# Questions tagged [equinox]

Questions about the moment when the Sun's declination is zero. A less strict definition is the day when the day and night are the same length.

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### Vernal equinox for 2550 BC - why is it on Apr 12 and not May 12th?

calculating exact date of vernal equinox in 2550BCE. A straight calc of 1 deg precession gives approx 45 days vernal equinox May 12 2550BCE ( from Mar 20 in 2024). other calc say Apr 12 2550BCE. ...
1 vote
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### What causes the short term variation in the length of a march-equinox year

By a "march-equinox year" I mean the time between successive march/vernal equinoxes. I understand that this is different from the mean tropical year, and I also understand why this would be ...
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### Determining Vernal Equinox by observation

Medieval society determined the date of Easter (or Passover) relative to the Full (or New) Moon on or after the (day of the) Vernal Equinox, where the state of the Moon was observed rather than ...
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### How can I build a gear-based mechanism to calculate solstices, equinoxes, and cross-quarter dates?

I realize there are boundless amounts of information as to the formulas to calculate solstices, equinoxes, and cross-quarter days, but I would like to design a gear-based mechanical solution for the ...
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### Calculating Earth-Sun distance only using trigonometry and terrestrial measurements

I have an interest in mathematics and astronomy, and my curiosity was piqued by the methodologies employed in contemporary solar distance measurement. Current practices use sophisticated instruments ...
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### Earth's rotation axis perpendicular to Sun-Earth line during equinox

Wikipedia says that the following happens during the equinox: More precisely, an equinox is traditionally defined as the time when the plane of Earth's equator passes through the geometric center of ...
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### Where is the vernal equinox of Mars located?

Dear Esteemed Astronomers, could you kindly advise how to find/derive the location of the Martian vernal equinox? Is it simply located at the cross product of Mars's north pole and the normal to Mars'...
1 vote
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### Where can I get historical data of tropical longitude of Delta Cancri

I need historical data of tropical longitude of Delta Cancri. Where can I get it. As of now it is approximately 128 Deg from Vernal Equinox. Where can I get it's historical data, say from 5000 BC to ...
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### What is the longitude (value) of the sub solar point of the vernal equinox?

I am new to this topic so correct me if I am wrong. But as I understand it, the vernal equinox point is a point on the celestial sphere where the ecliptic and celestial equator meets. When the sun's ...
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### What are the time periods of precession of equinoxes of other planets?

Earth has a time period of 26000 years approximately for precession of its equinoxes. What are the time periods of precession of equinoxes of other planets?
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### Why isn't Spring equinox on 1st of April?

AFAIK the summer and and winter solstices are based on longest day and longest night doesn't fall on 1st of Jan and July 1st because we consider the day to begin and end when the middle of the sun is ...
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### How the "precession of the ecliptic" can be distinguished from the "precession of the equator"?

The precession of the equinoxes (AKA general precession) is composed from two components: A dominate component which is caused by the the tilt of the Earth's axis - which is called "precession ...
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### How to determine sidereal time just looking at the picture?

I have this picture: I need to determine what sidereal time it is (in hours), or how long has it been since the upper culmination of the point of the vernal equinox. How can I do that?
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1 vote
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### Ecliptic and it's poles

Well, I have this picture and a couple of tasks to it: It is the Sun's path, a.k.a. ecliptic in autumn, 22-23 of September or on the day when autumnal equinox happens. I am to answer these questions: ...
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1 vote
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### At what time of the day of autumnal equinox the Sun's equatorial coordinates reach max value?

I'm trying to solve this task: At what point in time of autumnal equinox day, at sunrise or sunset will a) the declination of the Sun b) the right ascension of the Sun reach the maximum value? I'm ...
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### Are all the steps below leading this approximation for a zero-shadow latitude correct?

Here we calculate an approximation of the latitude which experiences zero shadow on (at the start of) a given day of a year. Assumptions: The Earth is spherical Its orbit is circular and the ...
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### Why is the sun not directly overhead at noon on the March equinox at N 0° 0' 0.00" E 0° 0' 0.00"?

I'm trying to teach my sons the difference between using azimuth-altitude coordinates and ecliptic coordinates. In the course of setting up some demonstrations, using Stellarium, I set my location to ...
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### Is the claim that the star Sirius does not precess like other stars true?

There is this popular claim on Internet that the star Sirius does not precess like other stars. The idea comes from mainly Greeks and Egyptians using Sirius to track seasons, as part of the Sothic ...
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I'm trying to solve this task: At what local time is the point of autumn equinox at its upper culmination three weeks after the vernal equinox? I think it has something to do with the sidereal day (...
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### In which town is the length of the day bigger on 23 of September?

I'm trying to solve this task: Moscow and Kazan are located on the same parallel. In which of them the length of the day is bigger on 23 of September? In Kazan the sun will rise earlier, because it'...
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### Lunisolar calendar: what phase was the moon Y years ago on the spring equinox? [closed]

How would I go around answering these questions (e.g., for $Y=10,000$ -- IOW, historical observations are not available and the time is too long ago to rely on a fixed calendar): How many days ago (\$...
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### Accuracy of calculating the vernal equinox?

Calculating the exact time of the vernal equinox is essential for many ephemeris calculations. NASA Goddard Institute for Space Studies uses (at least for some purposes) the following, rather short ...
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1 vote
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### Understanding date of astronomical events

I have a masters in chemistry but pretty much, a layman in astronomy. So, can you please explain to a novice like me, about this paragraph taken from the Wikipedia article on Makar Sankranti: There ...
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### Aligning the equinoxes to the cardinal points on a circular calendar

I want to draw a calendar as a circle, with the equinoxes at the top and bottom, and the solstices at the left and right. I am prepared to accept a little inaccuracy by having exactly 365 or 366 days ...
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### Why is the sunset at different times at the same longitude at equinox?

I happen to live in two places that are literally at the same longitude but several thousand kilometers apart from each other (~47°N and ~68°N). It's just a few days until equinox - and I assumed that ...
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1 vote
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### What direction does 0° in the J2000 epoch point towards on January 1, 2000?

Is 0 degrees in the J2000 Epoch 'right ascension' is a line from Earth to the sun on January 1, 2000? Or is 0 degrees the direction towards the ascending node 'equinox'? Which is NOT towards the Sun ...
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### Is the vernal equinox always in zenith somewhere on the equator?

I'm trying to visualize the satellit orbital parameter "right ascension of the ascending node" (RAAN). Measuring the angle from the vernal equinox to the point where the satellite's orbit crosses ...
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### Is the equinox absolutely true east everywhere?

I've read that only only in the equinoxes do you get to see the sun rise in the exact east regardless where you might happen to be. However I think this is not absolutely true everywhere, but only in ...
1 vote
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### Is this render of a ringed planet's shadow accurate?

I've noticed that when it's the equinox on this planet I'm making, the sun seems to cast a double shadow on the planet. How come the planet's ring seems to make two shadows with a space in between, ...
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### Sunrise and sunset times when compared between the vernal and autumnal equinoxes

Why is it that sunrise and sunset occur at different times when compared between the vernal and autumnal equinoxes? Consider Pontianak, Indonesia, 0 deg N, for simplicity. Timeanddate.com gives ...
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### How to calculate planetary positions hundreds or thousands of years ago

Wondering how to calculate the following in a general way: Sunrise/sunset Equinoxes/solstices Moon phases Planetary positions For hundreds to thousands of years ago. Wondering what the calculation ...
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### Why is the equinoxes only 2 days and not more?

The earth is tilted at 23.5 degrees which means that in the solstices the north/south hemisphere is more radiated by the sun and the tilt to the sun is maximum. In the equinoxes, the tilt is not ...
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### Is the equinox Sun always at the same zodiac sign in the ecliptic belt?

During the year the Sun moves along the ecliptic passing all the signs of the zodiac belt, in my understanding. This means that it takes ~1 month to go from one zodiac sign to the next. The ...
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### Solar azimuth for celtic fire festivals

I am pretty new to astronomy, and I have fallen down rabbit hole creating a new Metonic calendar in Javascript. As part of this I would like to calculate when the sun is half way between a solstice ...
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### Are the stars seen at the equator different throughout the year?

If you live on the equator and look up at the night's sky at, say, midnight, would you see a completely different sky on the 1st of January than you would on the 1st of July? I assume you would as ...
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### What is the great (platonic?) year?

How could a great year be explained in laymen's terms? According to the definition in NASA's Aerospace Dictionary it is a complete cycle of the equinoxes around the ecliptic that completes in 26000 ...
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### Why is the time between equinoxes different?

The path of the Earth around the sun is described as being an ellipse, with the Sun being at one of the foci of the ellipse. Since an ellipse, by definition, is symmetrical with respect to the foci, I ...
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