Talk:Vernal equinox

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[edit] Day longer than night

"Because the Sun is not a point but a ball, and because sunlight is refracted downwards by the atmosphere, the day by that definition is some 7 minutes or more longer, making the practical equality of day and night happen a few days towards the winter side."

Is there an astronomer who can set this straight? I claimed day was 26 minutes longer than night on the vernal equinox and cited the sunrise and sunset tables for London on 21 March 2005: 6:00 a.m. sunrise--6:13 p.m. sunset. That's signficantly more than 7 minutes.

And this theory that it is both the fact that the sun is a ball and that sunlight is refracted downwards by the atmosphere [so we're seeing the reflection of the sun in the horizon?] seems to me to be bogus. How much of the difference between day and night is caused by this refraction? I'll bet it's a second or third order effect.

Is there anyone in Wiki land who actually knows? 209.150.62.164 21:54, 17 September 2005

Quote from the arcticle day:

"The Sun is not a point, but has an apparent size of about 32 minutes of arc. Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. The difference in time depends on the angle at which the Sun rises and sets (itself a function of latitude), but amounts to almost seven minutes at least.

The 50″ mentioned is 0.5*32+34. So the refraction part is not a minor effect. The almost seven minutes happens on the equator (sun rising at 4 minutes per degree, counting both morning and evening effect). At higher latitude the Sun rises at an angle, so the time must be divided by the cosine of the latitude. London is at 51°30′, with a cosine of 0.62, leading to an increased duraction by 61%, amounting to about 11 minutes. Not so far from your mentioned (rounded) daylength from 06:00 till 18:13. Of course, the night gets shorter by the same amount, so indeed there is a factor two missing in the difference. I will correct. You will find a wealth information at [1]. −Woodstone 09:00, 18 September 2005 (UTC)

It took me a while to compose my response so Woodstone beat me, but I consider a few more effects, so I add my answer in full.
If the Sun moved at its average annual rate then it would take exactly half of a mean solar day or 12 hours for the center of the Sun to move from the eastern geometric or airless horizon to the western geometric or airless horizon at the equinox. The equation of time causes the Sun to arrive at its various cardinal points like noon or sunrise or sunset earlier or later by as much as 17 minutes at various times of the year. At the equinox the equation of time causes the sun to be east of the mean sun by about 7.5 minutes, causing it to arrive at both sunrise and sunset late. Because both are late by the same amount, there is no affect on the length of daylight. But the equation of time is actually the summation over the course of a year of many true solar days that can be as much as 30 seconds longer or shorter than a mean solar day. At the vernal equinox the true solar day is 24 hours 0 minutes 20 seconds long. This increases the amount of daylight by 10 seconds.
Sunlight moves faster in thin air than when the air is denser. This causes sunlight to bend away from the thin air above ground level and toward the denser air at ground level, allowing us to see objects like the Sun while they are still beyond and below the spherical surface of the Earth. At standard temperature and pressure, the refraction is 34' (arc minutes). The Sun is close enough to form an apparent disk of about half a degree. It must move half that or on average an additional 16' below the horizon before its upper limb or edge disappears below the horizon. At the equator the Sun moves vertically to the horizon, so the 34' of refraction increases the length of daylight by 2.27 minutes over that on an airless Earth at both sunrise and sunset, assuming that the eye of the observer is at sea level; while the 16' radius increases the length of daylight by another 1.06 minutes at both sunrise and sunset, assuming there are no mountains or other obstructions. Totaling sunrise and sunset increases the length of daylight by 6.67 minutes (6m 40s) at the equinox, but only at the equator.
At higher latitudes like London, the Sun crosses the horizon at an angle to the vertical equal to its latitude, here 51°32'. Imagine a right triangle at the eastern and western horizons. Its upper vertex touches the horizon due east or west, its right angle is 50' below the horizon, and its other vertex is also 50' below the horizon slightly north of due east or west. The angle at the upper vertex equals the latitude of the observer. The hypotenuse is longer than the vertical side by 1/cos(latitude), so here it is 82'51", taking 5.52 minutes to traverse, increasing the length of daylight at the equinox by 11.05 minutes when both sunrise and sunset are considered.
You cannot arbitrarily double the 13 minute late sunset to get 26 minutes. The difference between 6:01 am and 6:13 pm (both rounded to the nearest minute) is 12 hours and 12 minutes, which is quite close to my calculated increase in daylight of 11.05 minutes. Adding half of the true solar day for a true daylight increase of 11.22 minutes makes the comparison even better, which would be better yet if both sunrise and sunset were given to the second. I have ignored a few other effects, principally the fact that the triangles are really spherical, not planar, thus spherical trigonometry should be used to calculate the 'hypotenuse', which is part of a great circle.
— Joe Kress 20:39, 18 September 2005 (UTC)

Hi Joe, excellent detailed explanation. In your edit summary you deny the factor two. However your calculations confirm that the day is 11.22 minutes longer than 12 hours. Of course that makes the night 11.22 minutes shorter, so the day is 22.44 min longer than the night. That is the factor two I meant. −Woodstone 22:06, 18 September 2005 (UTC)

You're right. But because I misunderstood it, others would also, so I'll reword it. That also means that I should not have dismissed the doubling of 13 minutes by 209.150.62.164 as quickly as I did. — Joe Kress 04:52, 19 September 2005 (UTC)

[edit] Reversed naming in southern hemishere?

I was under the impression that the vernal equinox was always the one in March, and the one in September was always the invernal equinox or hibernal equinox, even in the Southern Hemisphere.   –radiojon 19:23, 2004 Sep 8 (UTC)

[edit] Old wives tale

I am not overjoyed by the use of the non-politically correct "old wives tale" here and I do not think it is appropriate to report every societal aberration in an encyclopedia. Perhaps if this has to be kept "old wives tale" could go to the Wiktionary and there be referred to "folk legend." Or a separate entry "Vernal Equinox Egg Myth" could be added. After all, some people expect earthquakes or weather anomalies when the planets line up a certain way, some people say it's bad luck to step on cracks in the sidewalk or have your path crossed by a black cat, and the Chinese, I have been told, believe it is a bad luck omen, a symbol of death, to give a clock as a gift. But that does not appear under "clocks" I believe; if it's anywhere it should be under "omen," "charm" or "harbinger of death" or the like Pdn 14:31, 25 Feb 2005 (UTC)

I personally think a mention about clocks in various cultures and what they might represent would be a worthy addition to any article about clocks. I'm going to mention this on the clock page... There isn't any mention of superstition about ladders on the ladder page, but black cat is all about the superstition regarding the felines. Your point about using the term "old wives' tale" is possibly a valid one, though... I'm not sure how correct it would be to use the term instead of something more clear. I dunno, I just see it as useful information, provided it's significant enough. I've never heard of this egg myth before, so it's likely unnecessary, but on the other hand, if most Chinese people feel strongly about the clock gift-giving, I feel that might be worthy of inclusion. After all, Timbits have a page despite some people wanting to remove it, since the little things are so immensely a part of Canadian culture they might as well be mentioned in the Canada article (if they aren't already)... JC 09:14, 15 February 2006 (UTC)
Please allow political correctness to die the undignified death it deserves. Anyone offended by the term "old wives' tale" has too much time on their hands thx1138 16:40, 20 April 2006 (UTC)

[edit] Practical effects

I would like to add the following text, copied from Earth Day, to the end of the introduction

It is the precise moment that spring begins in the Northern Hemisphere and autumn in the Southern Hemisphere. At this global moment, night and day are equal all over the world, the sun sets at the South Pole and rises at the North Pole and anyone standing on the equator at noon will not cast a shadow.

--Etimbo | Talk 11:12, 20 Mar 2005 (UTC)

You are supposed to be bold when editing Wikipedia, so go ahead. If any other editor does not like it, they will modify it. — Joe Kress 20:54, Mar 20, 2005 (UTC)
I normally like to give people advanced warning before changing the introduction to articles so that I don't ruffle feathers; but you have spurred me into action and I have made the change! --Etimbo | Talk 12:37, 21 Mar 2005 (UTC)

[edit] Please correct misconceptions

The orbital diagram is beautiful, but misleading. It suggests that the earth's axis points in a different direction at different seasons of the year. It does not. The earth's axis should be tipped in the same direction for both the summer and winter solstice illustrations. You should then change the direction from which the sunshine comes from -- from the right in one, and from the left in the other -- just as it actually does when the earth shifts position by 180 degrees in its orbit. I'd omit the illustration for the equinoxes entirely. Anything you would draw would suggest that the earth's axis is oriented straight up and down, which is of course not correct. In reality, the sunlight would be streaming perpendicular to the page, which is not possible to draw in a two-dimensional illustration. User:BartBenjamin

It would also probably be better to use the terms "December Solstice" and "June Solstice" instead of "winter" and "summer." That way, it is correct for all hemispheres. That is how the International Planetarium Society solves this dilemma for its members located in both hemispheres. User:BartBenjamin

[edit] Picture

    • Someone should probably try and find a picture that isn't just a reuse from equinox--Hello'from'SPACE 00:29, 15 November 2005 (UTC)

[edit] Merge

I suggest that March_equinox merged in this article. Most of information seems to be repeatitive. Mahanchian 11:25, 20 March 2006 (UTC)

Someone created March equinox from this article because he did not like this article's "northern hemisphere bias". — Joe Kress 09:36, 21 March 2006 (UTC)
I would like to suggest MARCH EQUINOX the main article (hemispherically unbiased), redirected to by FIRST POINT OF ARIES and VERNAL EQUINOX; likewise SEPTEMBER EQUINOX redirect to by FIRST POINT OF LIBRA and AUTUMNAL EQUINOX. Or even better: one EQUINOX article (calendrically unbiased) for all of them. --Tauʻolunga 08:01, 1 July 2006 (UTC)
The problem I have with any merge is that in astronomy the northern vernal equinox is the origin of ecliptic longitude (0°), right ascension (0h), and the sidereal day (0h). The southern vernal equinox has no special significance in astronomy. The same applies to astrology. Although the First point of Aries conicides with the northern vernal equinox in western astronomy, in the Hindu calendar it now occurs in April because that calendar ignores precession. — Joe Kress 19:50, 2 July 2006 (UTC)
The article would split up in at least 4 main parts: heliocentric view, from which the season explanation would follow; geocentric view to do the same thing; geocentric view focusing on the rest (i.e. coordinate systems); others (astrology, Hindu,…) --Tauʻolunga 21:11, 2 July 2006 (UTC)
I don't understand the your distinction between heliocentric and geocentric seasons. — Joe Kress 00:25, 5 July 2006 (UTC)
I did not say h & g seasons, I said h & g viewpoints to explain the seaons. I found often that the drawing of the Earth with its titled axis in its orbit around the Sun is easier to comprehend by the students than the drawing of the ecliptic on the celestial sphere. --Tauʻolunga 02:35, 5 July 2006 (UTC)

[edit] Reason for variation of date

Could someone who knows why add why the date of the equinox changes irregularily (sometimes the 20th, sometimes the 21st)? The already mentioned precession is an extremely minor effect and cannot be responsible for this. I suspect the reason is the moon or maybe even jupiter but I'm definitely not sure enough of that to add it to the article. —Preceding unsigned comment added by 80.219.217.200 (talk • contribs)

I have the same problem. All thoughout my youth and early adulthood the Equinox always occurred on the 21st. One of the reasons this stuck with me so well is that I then was able to state that my birth occurred on the first full day of Spring ie the 22nd. I would be extremely interested in hearing how and when this change happened. Michael Cunningham, Milwaukee WI

See Gregorian calendar#calendar seasonal error for a graph showing the variability in the date of the astronomical winter solstice. The astronomical vernal equinox has a similar variability due to the extra quarter day in the length of the vernal equinox year (365.2424 days) and the leap days of the Gregorian calendar used to account for it, both those every four years and the centurial leap days skipped. But only the astronomical equinox varies—the ecclesiastical vernal equinox has (almost) always been on March 21, and still is there. The latter is used to determine Easter (see computus). — Joe Kress 09:36, 21 March 2006 (UTC)