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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1answer
46 views

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 ...
22
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3answers
5k views

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 ...
-2
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1answer
164 views

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 ...
3
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1answer
75 views

Task about equinox and upper culmination

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 (...
1
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1answer
95 views

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'...
2
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1answer
56 views

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 ($...
2
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2answers
273 views

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 ...
1
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1answer
63 views

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 ...
6
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4answers
358 views

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 ...
8
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1answer
301 views

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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1answer
52 views

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 ...
2
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1answer
191 views

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 ...
3
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2answers
270 views

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 ...
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4answers
157 views

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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2answers
166 views

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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0answers
152 views

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 ...
2
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4answers
503 views

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 ...
3
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4answers
472 views

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 ...
2
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1answer
139 views

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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1answer
156 views

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 ...
3
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1answer
329 views

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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2answers
544 views

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 ...
3
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1answer
202 views

What's a good model for the vernal equinox year? Can it increase forever?

Duncan Steel (2002) makes an interesting argument that the notion of year is (1) somewhat arbitrary and, more interestingly (2) one notion of year can increase while another decreases. His examples ...
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1answer
1k views

How was the time of equinox measured in ancient times?

In the book "The Crime of Claudius Ptolemy" by Robert Newton, on page 81, it reads as follows: When we measure the time of a solstice, we measure the time when the meridian elevation angle of the ...