Equinox, Equilux, and Twilight Times
On or around 21 March each year it is the Spring Equinox in the Northern Hemisphere, the day when eggs can be stood on their ends, when the Sun crosses the celestial equator, and when night and day are equal length the world over.
But this last part isn’t entirely true, for a variety of reasons.
Equinox
Equinox is latin for aequus (equal) and nox (night) meaning, roughly, “equal night”. The moment of the equinox is defined as the point at which the centre of the Sun’s disk crosses an imaginary line in the sky called the celestial equator, the projection of the Earth’s equator out into space.
The Sun (and the Moon and all the planets) move along a line in the sky called the ecliptic, the projection of the disk of the solar system out into space. These two lines, the equator and the ecliptic, cirlce the sky, and because the Earth’s axis is tilted at 23.5 degrees the angle between the equator and the ecliptic is 23.5 degrees, and the two circles meet at only two points, called equinoctial points.
Over the course of the year the Sun, as seen from Earth, appears to make one complete circuit around the ecliptic, as the Earth in fact orbits the Sun. And so on two days each year the Sun’s path crosses the equator. This means a number of things:
1. that an observer at the equator will see the Sun directly overhead at mid-day on the equinoxes
2. that the Sun will rise due east and set due west on the equinoxes (on all other days the Sun will rise either north or south of east, and set north or south of west)
3. the length of day and night are nearly equal
On this last point, they are not exactly equal, for two reasons:
1. the Sun appears as a disk in the sky with a radius of around 16 arcminutes, and so the top of the Sun appears to rise while the centre of the disk is still below the horizon, and the instant of the equinox is measured with respect to the Sun’s centre, and
2. the Sun’s light is bent, or refracted, in the Earth’s atmosphere, so that rays from the Sun can light you up even before the Sun rises, and keep you lit after it sets, with the degree of refaction being around 34 arcminutes
These two factors combine to mean that the Sun will appear to have “risen” when the centre of the disk is still 50 arcminutes (16 + 34) below the horizon, making the amount of daylight longer than the expected 12 hours. How much longer depends on where on Earth you are, but in the UK the length of the day is approx. 12 hours 10 minutes, rather than exactly 12 hours.
Equilux
Because of this effect, the days on which the length of day and night are exactly equal, called the equilux, occur a few days before the spring equinox and a few days after the autumn equinox. This date will vary depending on where on Earth you are, and indeed equiluxes do not occur at all close to the equator, whereas the equinox is a fixed instant in time.
Twilight
But the story doesn’t end there. Even on the days of equilux, the sky will have been bright for some time before the first rays of the Sun hit you, and will remain bright for some time after the last rays disappear from view. This time of day is called twilight, starting at dawn and ending at dusk.
There are, in fact, three kinds of twilight.
Civil twilight is what most people mean when they talk about twilight. It starts in the morning when the centre of the Sun’s disk is 6 degrees below the horizon, and ends at sunrise. In the evening civil twilight stars at sunset and continues until the centre of the Sun’s disk is 6 degrees below the horizon. During civil twilights the sky is still bright enough that, in general, artificial illumination will not be needed when doing things outside. In reality though, most councils switch lights on a fixed time after sunset (say 30 minutes) and turn them off a fixed time before sunrise, rather than relying on the Sun’s angular distance below the horizon.
Nautical twilight is when the Sun’s disk is between 6 and 12 degrees below the horizon. During nautical twilight it is still possible to distinguish the sky from the distant horizon when at sea, thus allowing sailors to take measurements of bright stars against the horizon. Most of us would consider this “dark”, but it is still technicaly twilight.
Astronomical twilight is when the Sun’s disk is between 12 and 18 degrees below the horizon. During astromical twilight you’ll no longer be able to tell the sky from the distant horizon when at sea, but crucially for astronomers, there is still light in the sky. Not much – indeed most of us would say it’s properly night time at this point – but the faintest objects in the sky, such as nebulae and very dim stars, will only be visible after astronomical twilight ends. Also, if you intend to measure how dark the sky is using a Sky Quality Metre, say, it is important to wait till after astronomical twilight.
So how much daytime does this twilight add? Only during civil twilight can the sky really be considered “light”, and so we can say that the day begins (at the point we would call dawn) at the start of morning civil twilight, and ends, (at the point we would call dusk) at the end of evening civil twilight. This means that on the Spring Equinox, the “day” will be around 13 hours and 15 minutes long in the UK (depending on where you are).
The date at which the “day” (including dawn and dusk) is 12 hours long in the UK (and therefore when we had equal amounts of daytime and nighttime) occurs around 01 or 02 March, over two weeks before the so-called equinox!
To find out your local sunrise / sunset / twilight times visit www.timeanddate.com
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Dark Sky Diary 
very good explanation
interesting for a non-geek as well
What a lot of meaningless religious blathering! Your fixation on that collection of stories by near-illiterates from 1700 to 1900 years ago prevents you from learning anything meaningful from contemporary sources.
And you waste the time of those of us foolish enough to try to understand what your driving at.
I agree. For anyone who wants the scientific facts for the arguement…….this is the info missing from the blathering. But you can only lead a horse to water- http://de-fact-o.com/fact_read.php?id=100
How does an illiterate person(s) write using a large vocabulary? Can you explain that one please. That statement is totally illogical.
EQUINOX EQUATOR
The traditional calendar places the “Equator” in the wrong place!
The Equator is an imaginary line around the CENTER of the Earth.
The traditional calendar does not have the Equator in the center of the Earth but has it about a month above the true Equator therefore the Center of the Sun crosses the Center of the true Equator, causing a true Equinox of equal light and darkness, about a month BEFORE the traditional Equinox. This causes the seasons to be off about a month.
This can be scientifically proven by shinning a light in the center of a spear. You can use a tennis ball or a globe or any Spear shaped object. When the light is shined in the center of the ball, it will cause ½ of the ball to be light and the other ½ dark, producing EQUAL light and darkness on the ball. The same is true with any Spear “including the Earth”. When the light is moved up or down from the center of the ball, you will notice the shadows changing which represents the seasons because the length of the light and darkness is what causes the seasons. The reason you do not have equal light and darkness at the traditional equinox on March 20 is BECAUSE the Sun DOES NOT shine on the Center of the Earth, or Equator, at that time.
Conclusion
The Equinox is when you have equal amounts of light and darkness and this is “only possible” when the center of the Sun is at the “center” of the Earth, or true Equator. This nature made Equinox happens twice each year, around February 16 and October 26 here in Georgia, which is on the same agricultural plain as Israel.
You can verify this in any Farmer’s almanac. All you have to do is find when the length of the day is 11 hours from sunrise to sunset and this will give you about 30 minutes of light before the Sun is at the horizon and another 30 minutes after the Sun is below the horizon in the West, making a total of 12 hours of light and by default there will be 12 hours of darkness. If you use the traditional 12 hours from sunrise to sunset, you will have 13 hours of light and 11 hours of darkness unless you count the morning and evening light as darkness as they do, which I believe to be unscriptural. Again, this happens about a month before the traditional equinox. Remember it is the light of the Sun shining on the equator, or the center of the Earth, that causes the equal light and darkness/equinox and this does not happen on March 20 therefore the traditional equinox and equator is bogus and should not be used to set the Almighty’s holy feast days each year.
The Roman calendar thinks to change the seasons, Daniel 7:25, by creating a bogus equator above the true equator that the light reveals. The light makes manifest truth, both natural and spiritual. Only the light of the sun can show where the true equator is. Do the experiment with the ball for yourself. The Scripture speaks negative of moving landmarks and I believe the same could apply here.
sphere, not spear…
The equilux and equinox are being confused. Equilux is the day of approximately equal light and dark hours. Because of the refraction of the sun’s light by the atmosphere, this day is observed to occur on the 16th of March – the sun appears to rise above the horizon before it physically rises. Equilux is a modern measure that we are only able to make because we have clocks and can measure the time of the daylight. Before clocks, the day of equilux did not exist.
Ancient peoples only had access to instruments such as sundials that allowed them to measure the angle of the sun with respect to their location on earth. The people at that time used the term equinox (equal night), and they understood it to be the day of equal day and night. Now that we have modern clocks, we can see that the day on which we observe equal hours of light and dark (due to atmospheric refraction) is a few days different than the astronomical event of equinox. You can observe equinox with a sundial, but a sundial will not tell you the time of equilux.
Regardless of the precise physical location of the equator in Equador, the timing of the astonomical equinox is unaffected. At the time of the equinox, the earth’s pole is aligned with the sun such that there are approximately 12 hours of daylight across the entire earth (north pole to south pole).
CN
Very well written… I have been trying to tell people this for a while now…
I would like to know more on the difference between the Equinox and the Equilux
Very instructive..good post
Nice!!!!
According to timeanddate.com, in London around the December solstice (shortest day of the year) although the sun rises at 8am and sets at 4pm, astronomical twilight starts at 6am GMT and ends at 6pm GMT, meaning that if “twilight” is considered as daytime, day and night would be equal length on the shortest day (i.e. the time there is light in the sky and the time there is not light in the sky) will be EQUAL!!
Mc kay Tims ; Mr. Arnold Bowen is biblically correct, and after multiple year studies we have found that the stars, trees, animal kingdom, man, as well as the earth orbit, witness to the Greatness of THE CREATOR’S DESIGN. The facts of nature as well as the division of light and darkness in the book of Genesis is absolute truth.
The only fact that we may differ on is that after our studies we find the True Equinox, to fall every year on the night of February 14th except when on years that the Gregorian calendar [which is not the creators account of earth time] adds an extra day in February to adjust the mishap of Mans own attempt to [know more than YHWH] create his own commercial and religious time keeping [calendar]. When the added day is inserted the True Equinox night will fall on February 15th.
The mere mention of todays definition; of the March 20th [false equal night]_
of being IMAGINARY LINES.,though the EARTH, and through the SUN, projected out into space where they intersect is preposterous on it’s face to think that men living 2 or 3 thousand years ago would have concocted such a thought.
Men who studied these things looked to nature, shadow poles for measuring length of day, the knowledge that winter and summer was not about temperature but the length of each day compared to length of each night, today there is a term for this daily ossilation in length of day and night called PHOTOPERIODISM which is a scientific fact of nature which [YHWH created] there is no refuting.
Man has always rebelled against the creators laws through-out history and are even so doing today. The Gregorian calendar is not to be trusted over impiricale evidence of nature and natures CREATOR.
To find the true nature of anything you must always investigate the WHO, WHAT , WHEN, and the WHY and the underlying prevailing of the times. The calendar we have is a product of multiple manipulations and reinventions by men but who never consulted the real evidence but chose their own agendas, such as power, wealth,[whether personal, political, as well as religious control of people]. They used pagan practices to deceive and created fear in the hearts of men as well as the inquisitions, torture ,death by burning alive, and implement and evils to horrible to mention here. This is the nature of the who, I will leave the rest to those few who will study-out a matter. THE TRUE EQUINOX IS IN FEBRUARY NOT MARCH . Equinoctial Points is not the same as the Equal night or equinox, by definition. The TRUE EQUINOX [night] will cause the day before it to have exactly 11hrs of light and that night to have exactly 11hrs of darkness; this will be the day winter ends and spring begins, opposite at the fall equinox.
I think being “biblically correct” is utterly meaningless in this instance. Your obsession with the content of a book written by bronze- and iron-age peoples has no bearing on the veracity of our modern understanding of the universe.
from McKay Tims MR.Owen;
: I would like to take this opportunity to speak to this matter about the difference of modern understandings of the universe, of which I admire todays knowledge and understandings. First thank you for your excellent presentation on this web site and the humor and the serious studies I am sure you have undertaken in astronomy.
I want to address first off that I would not in any way disrespect you or your understandings, and want to assure you and all your readers that;[ I am not coming from some argumentative point of view, or some smart- a_ _ attitude], but that there are, real distinctions between the Modern date of March 20th, as the night for which seasons change,or the day of equalization occurs.
Well before there was any Modern Knowledge there was ancient knowledge, although much of it has been proven to be incorrect there is large base of old knowledge, that today, is still the basics principles for which modern understandings rest upon.
The modern calendar should be a mirror of the Earths progression around our sun,but we find this is not the case. The Gregorian calendar is at its heart a Religious,Comercial institution since 1582ad that does not take into account the, precession of the equinox’s,which is an astronomical term of events, as well as the length of light and darkness which vary daily and seasonally throughout the year. The elliptic orbit of earth as well as, the sun being off-set[ by some 3.1 million miles out of center of the orbit ] requires there being an unequal number of days on the winter side of the sun and the summer side of the sun. The modern calendar[Gregorian] presents a false understanding that there 4 equal seasons in the year, 91 days in three quadrants of the orbit of the Earth around the sun and one quadrant composed of 92 day; For this above description of the Modern calendar [Gregorian] to be true the Earths orbit would be a perfect circle and the sun would be required to be placed in the absolute center of the orbital circle, which none of this is really true in our yearly trip around the sun.
Back to the history of our modern calendar [Gregorian] which came from the corrections of a more ancient [Julian] Pope Gregory realigned the date of March 20-21 to the edits issued at the counsel of Nice in 325ad. The calendar today is not based upon movement of our planet or any other star or our moon. the understanding of time is the movement of heavenly bodies, planets, earth in the solar system, not some man or institution.
We must honestly consider that DEFINITIONS of WORDS as well TERMS and NAMES change over time as well as cultures and languages even in our life time words and terms take on different connotations and definitions.
The Modern Explanation or Defining of Equinox as, lines placed perfectly through the center of the earth and perfectly through the center of the sun extending and intersecting out in space Would be as FOREIGN to men [ as you put it UTTER MEANINGLESS] living in ancient times in Greece or Rome even in 325ad, even until math and telescopes was fully developed and man could come to these[ scientific modern terms ].
When we take an open minded look at the time, When , Where, and Why, we see that men were looking at something totally different in ancient times that was occurring around them in nature for which they could measure when the seasons were changing and thus when the old year ended and the New Year began. Men from ancient times worshiped and planted their crops by this event called Equinox in Greek times and much earlier times, but it was the Greeks that termed the [Equal-night] or Equinox. Before there was a 12 month year the month we call March was the first month of the year, after January and February was added to the calendar count February was the 12month.
When we understand that when something old ends [such as a year] immediately the [new year should begin], also when a planet or heavenly body completes it’s revolution around its star the old year ends and the new year begins. This was what the ancients was looking to find each year. This is the event happening in nature for which got the term [equinox] in ancient times . When we studied this out to it’s conclusion we found that the actual defining of WINTER is LONG NIGHTS, and SHORT DAYS —- verses —- SHORT NIGHTS and LONG DAYS OF SUMMER, there is Two specific NIGHTS in the year where these SWAP-PLACES, we found them in the ancient calendar month [12 February and the 8th month October]. This is also what we find in the true path of our Earth around the sun that Truthfully there is eight[8] months on the summer side of the sun and four[4] months on the winter side of the sun in the northern hemisphere.
When the catholic church entered into calendar making and changing to always align their Easter holiday in 325ad to never align to the Hebrew Passover, over the years between 325ad and 1582ad the Equinox had been moved at least 3 different dates,MARCH 4th, March 14th, March 25th. Pope Gregory LAST moved it back to March 20th to date set in 325ad at council of Nice.
We can tell that the date March 20th is arbitrary and was not set by any astronomical event and is not a day with equal light and equal darkness in the same day.
We should understand there will never be a day in the year where there is 12 hours of light in the same 24hr day period 12hrs darkness because of twilight period each day which lengthens or shortens through-out the year.
I know some of you think this could not be true, but once the Earth was thought flat, and the earth revolved around the sun which was proven false but the ancient law of gravity is still un-refutable,the law of light and darkness called[ modern term photoperiodism] and the law E=MC squared on which most all modern science hangs may prove faulty,the jury is still out. THANK YOU SIR AND ALL FOR YOUR TIME AND CONSIDERATION MAY ALL WHO STUDY AND PONDER THIS GET A BLESSING FOR WHICH I STILL BELIVE.
Hi again, thanks for your detailed reply, and sorry for my earlier snippy response to your last posting.
That said, there are a few facts which need stated after which your argument may no longer be necessary:
1. The astronomical equinoxes and solstices are very specifically about where the sun is in our sky, and by association about the amount of time the sun spends above the horizon, and are absolutely NOT about seasons, when summer starts and ends etc. The rough timings of the four seasons can roughly be guessed from the astronomical timings but you’re right that traditionally the start and end of seasons can be judged by happenings on earth and how they relate to what’s in the sky at that time.
2. The instant of the equinoxes is defined as the moment when the sun passes from the north to the south celestial spheres (or vice versa). This is an irrefutable observation.
3. The instant of the solstices is defined as the moment when the sun reaches it’s furthest north or south of the celestial equator, another irrefutable observation.
4. On the equinoxes the centre of the sun’s disk is above the horizon for exactly 12 hours, and below for exactly 12 hours. This is also irrefutable.
5. The hours before sunrise and after sunset are bright, extending the length of “daylight” by up to three hours in the UK, giving something like 15 hours of daylight and 9 hours of darkness at the equinoxes.
6. There are dates at which the hours of observed daylight and darkness are equal and these are known as the Equilux.
7. While some ancient cultures would have noted these days, it was actual observation of the sun and moon that governed their observances and so the dates on which the sun rose and set due E and W and spent 12 hours above the horizon were far more important, as we can see from virtually every astronomically aligned ancient monument.
Perhaps if you’re inclined I reply you can state which of the above points you disagree with and then outline your own points concisely in number form so that I can address them one at a time.
Kind regards
Steve
Craig n you are very right and i have been researching it for years ,the real equinox/ equilux is the 16th – it is every 16th march of every year – it does not neglect or delay a single day ,years after years ,jerusalem time is the right ,the centre of the earth
What stance do you take on solar centric or geo centric? Great article you explain things in a way that a novice can understand.
CynnLynn
The following supports your assessment of the THE TRUE EQUINOX around March 1st or 2nd. I believe the original EQUINOX of the ancient was understood to be equal night. This is confirmed by the writings of a Levite priest by the name of Philo, who actually lived at the time of our Savior, see quotes from Philo, who was an eyewitness as to how they understood equinox during the time the Temple in Jerusalem was still standing, at the end of this response.
The Scripture itself in Exodus 12:2 speaks of the first month of the year/revolution which is in the springtime. The Hebrew word for year is “revolution” and the only noticeable conjunction point or cycle of the year/revolution in the springtime would be at the equal day and night regardless of what was going on at the equator two or three weeks later when you have 12 hours from sunrise to sunset which is not equal days and equal nights and would not fit the first day of the year in creation when he divided the light from darkness equally.
We have a conjunction point for the rebuilding of the new moon and the day, why not a conjunction point for the New Year and where is it at? Does the New Year have a conjunction point we can see without using the Earth same as the new moon and day does? Could the conjunction point be equal day and night or equal light and darkness? Could it be that the evening and morning was the divider of light and darkness? Can the phrase “let there be light/illumination” in Genesis 1: be referring to the light of civil twilight etc, before you actually see the sunrise? The new moon light begins to rebuild before we see the moon, why can’t the light of the new day began to rebuild before we see the Sun and why can’t the new year or revolution of the yearly cycle begin before the 12 hours from sunrise to sunset at the equator???
Case for Equal Night Equinox.
We need to find out if a 12hr day begins at dawn or sunrise. How was the day part of the 24 hour day measured?
Our Savior said, “Are there not 12 hours in a day?” The question is does these 12 hours in a day include the “morning/dawn” before sunrise as the sun pushes up the dawn, or are they measured from sunrise to sunset excluding the dawn/morning that is pushed up by the sun? Remember the sun is following the dawn/morning, how can you not count it??? Why wait till the sun peeks at the horizon before counting the light?
Philo
THE SPECIAL LAWS, IV*
{**Yonge’s title, A Treatise on Circumcision.}
XLII. (233) “Nature, therefore, has marked out those PERIODS IN EVERY YEAR, which are called the EQUINOXES, from the STATE OF THINGS WHICH EXIST AT THAT TIME, namely, the spring and the autumnal EQUINOX, with such DISTINCTNESS, that EVEN THE MOST ILLITERATE persons are aware of the EQUALITY which THEN EXISTS between the EXTENT of the DAYS and of the NIGHTS.”
ON THE CREATION IX. (32) “Moses is right also when he says, that “darkness was over the face of the abyss.”….God put a wall between them and separated them, well knowing their opposite characters, and the enmity existing between their natures. In order, therefore, that they might not war against one another from being continually brought in contact, so that war would prevail instead of peace, God, burning want of order into order, did not only separate light and darkness, but did also place boundaries in the middle of the space between the two, by which he separated the extremities of each. For if they had approximated they must have produced confusion, preparing for the contest, for the supremacy, with great and unextinguishable rivalry, if boundaries established between them had not separated them and prevented them from clashing together, (34) and these boundaries are evening and morning; the one of which heralds in the good tidings that the sun is about to rise, gently dissipating the darkness: and evening comes on as the sun sets, receiving gently the collective approach of darkness. And these, I mean morning and evening, “(35) But when light came, and darkness retreated and yielded to it, “and” boundaries were set in the space between the two, namely, evening and morning, then of necessity the measure of time was immediately perfected, which also the Creator called “day…..”
WHO IS THE HEIR OF DIVINE THINGS XXVIII. (141) “But since Moses not only uses the expression, “he divided,” but says further, “he divided in the midst,” it is necessary to say a few words on the subject of equal divisions; for that which is divided skilfully just in the middle makes two equal divisions. (142) And no man could ever possibly divide anything into two exactly equal parts; but it is inevitable that one of the divisions must fall a little short, or exceed a little, if not much, at all events by a small quantity, in every instance, which indeed escapes the perception of our outward senses which attend only to the larger and more tangible burdens of nature and custom, but which are unable to comprehend atoms and indivisible things. (143) But it is established by the incorruptible word of truth that there is nothing equal in inequality. God alone therefore seems to be exactly just, and to be the only being able to divide in the middle bodies and things, in such a manner that none of the divisions shall be greater or less than the other by the smallest and most indivisible portion, and he alone is able to attain to sublime and perfect equality….
WHO IS THE HEIR OF DIVINE THINGS XXIX. (146) These things being thus previously sketched out, see now how God, dividing things in the middle, has divided them into equal portions according to all the ideas of equality which occur in the creation of the universe. He has divided the heavy things so as to make them equal in number to the light ones, two to two; that is to say, so that the earth and the water, being things of weight, are equal in number to those which are by nature light, air, and fire. Again, he has made one equal to one, the driest thing to the wettest thing, the earth to the water; and the coldest thing to the hottest thing, the air to the fire. So, in the same manner, he had divided light from darkness, and day from night, and summer from winter, and autumn from spring; and so on. (147) Again, he has divided things so as to make his divisions equal in point of magnitude; such as the parallel cycles in heaven, and those which belong to the equinoxes both of spring and autumn, and those which belong to the winter and summer solstice….. (148) Now the “divisions of time” are equal in point of length, the longest day being equal to the longest night, and the shortest day being equal to the shortest night, and the mean length of day to the mean length of night. And the equal magnitude of other days and nights appears to be indicated chiefly by the equinoxes. (149) From the spring equinox to the summer solstice, day receives an addition to its length, and night, on the other hand, submits to a diminution; until the longest day and the shortest night are both completed. And then after the summer solstice the sun, turning back again the same road, neither more quickly nor more slowly than he advanced, but always preserving the same difference in the same manner, having a constantly equal arrangement, proceeds on till the autumnal equinox; and then, having made day and night both equal, begins to increase the length of the night, diminishing the day until the time of the winter solstice.”
(150) “And when it has made the night the longest night, and the day the shortest day, then returning back again and adopting the same distances as before, he again comes to the spring equinox. Thus the differences of time which appear to be unequal, do in reality possess a perfect equality in respect of magnitude, not indeed at the same seasons, but at different seasons of the year.
Again, notice carefully how he is tracing length of day and night and how they are equal length at the two equinoxes. Notice also, he is speaking specifically ABOUT THE DIVISIONS OF TIME and “begins” at the vernal equinox explaining how that at that time the days begin to grow longer and the nights shorter until the summer solstice and then the nights longer and the days shorter until the autumnal equinox at which time the days and nights are equal again. He then explains how the nights began to grow longer and the days shorter until the winter solstice at which time they reverse and the day start growing longer and the nights shorter until they become equal again at the spring equinox AND THEN THE CYCLE ENDS or is completed. He had a good chance to say something about the equator in measuring the cycle but his understanding of the word equinox had to do with equal night and day. He does mention equator when talking on another subject, of temperature, but not time. Remember also that Philo, who was a Levite priest, would have had to know when the year, month, and day “cycles” end in order to carry on the duties of the priesthood. This is in harmony with Exodus 12:2 concerning the revolution or cycle of the year. I’m not sure about this next quote.
According to the Book of Enoch, this is the Last Day of the Solar Year, Enoch chapter 72. Man has erroneously made the Vernal Equinox the First Day of the Year. This throws off the Solar Calendar by one day. The First Day of the Solar Year begins when the Light becomes longer than the Darkness following the Vernal Equinox.
I don’t think this contradicts what Philo says.
THE FIFTH FESTIVAL
XXVIII. (150) And there is another festival combined with the feast of the passover, having a use of food different from the usual one, and not customary; the use, namely, of unleavened bread, from which it derives its name. And there are two accounts given of this festival, the one peculiar to the nation, on account of the migration already described; the other a common one, in accordance with conformity to nature and with the harmony of the whole world. And we must consider how accurate the hypothesis is. This month, being the seventh both in number and order, according to the revolutions of the sun, is the first in power; (151) on which account it is also called the first in the sacred scriptures. And the reason, as I imagine, is as follows. The vernal equinox is an “imitation” and “representation” of that beginning in accordance with which this world was created. Accordingly, every year, God reminds men of the creation of the world, and with this view puts forward the spring, in which season all plants flourish and bloom; note: how could Philo say this unless he understood the first day of the year in creation is at the equinox??? (152) for which reason this is very correctly set down in the law as the first month, since, in a manner, it may be said to be an impression of the first beginning of all, being stamped by it as by an”
QUESTIONS AND ANSWERS ON GENESIS 111, 3
In the divisions of earth there is a vast quantity of dry continent, islands and peninsulas. Water is divided into sea, rivers, and lakes; and the air into the two equinoxes, the vernal and the autumnal; and they may be taken as one, for they have an equal proportion of day and night, and accordingly the equinoxes are neither hot nor cold. Add to these the changes of summer and winter, for the sun is borne through those three circles into the seasons of summer, winter, and the equinoxes.”
There are more but this should be enough for us to take a second look to how they understood the true equinox.