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35 votes
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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"?

It never is exactly, directly overhead on the vernal equinox at the Greenwich meridian. You know that it happens on different days in different years so it must be happening at different times on ...
stretch's user avatar
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25 votes

Why is the sun not directly overhead at noon on the March equinox at N 0° 0' 0.00" E 0° 0' 0.00"?

In addition to the March 2021 equinox occurring 2.4 hours before 12:00 UTC as stretch explains, you found a maximum altitude at 12:07:25 UTC because apparent solar time was 7.4 minutes behind mean ...
Mike G's user avatar
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20 votes

Why is the sun not directly overhead at noon on the March equinox at N 0° 0' 0.00" E 0° 0' 0.00"?

The time of the equinox, and the Sun being over the 0° meridian are determined by different motions: The Earth spins around its axis. The Sun crosses over the 0° meridian once every solar day, when ...
roeland's user avatar
  • 309
15 votes
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Why isn't Spring equinox on 1st of April?

The calendar is designed so the 21st of March is set to the spring Equinox, and everything else is calculated from there. So the reason that the equinox doesn't occur on the first of April is boring. ...
James K's user avatar
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14 votes

Why is the time between equinoxes different?

This is because the summer and winter solstices (approx. June 21st and December 21st) do not correspond to the aphelion and perihelion (approx. July 5th and January 4th). Therefore, the average ...
Glorfindel's user avatar
  • 4,800
14 votes
Accepted

Earth's rotation axis perpendicular to Sun-Earth line during equinox

The Sun-Earth line is indeed the line connecting the centers of the sun and the earth, which exists on the ecliptic plane. At the earth's location, there is exactly one line perpendicular to the ...
Nuclear Hoagie's user avatar
11 votes
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Why is the sunset at different times at the same longitude at equinox?

The only idea I have that it might be due to the definition of sunset. The instance of sunrise occurs when the first bit of the Sun becomes visible at the horizon, while the instance of sunset occurs ...
David Hammen's user avatar
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10 votes
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What is the longitude (value) of the sub solar point of the vernal equinox?

Let's take the easy way and let's look up the time for the equinox this year: my astronomical yearbook tells me that the autumn equinox happens at exactly 8:50h MESZ on 23 September 2023. MESZ is UTC+...
planetmaker's user avatar
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8 votes

Why is the equinoxes only 2 days and not more?

First, don't think that equinox is when the day and night are the same. It is the moment, when the sun's declination is zero, or when the sun is directly over the equator. Let me explain ... Earth is ...
User123's user avatar
  • 2,879
7 votes

Is the equinox Sun always at the same zodiac sign in the ecliptic belt?

As you said, the Sun takes a year to cycle the Zodiac, and it takes a year between two spring or autumn equinoxes. Both statements are true, but "year" in them means two different things. It takes a ...
Pere's user avatar
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6 votes
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Accuracy of calculating the vernal equinox?

The climate modelers' VERNAL is equivalent to this formula, where Y is an integer, ΔY = Y - 2000, and JD0 is epoch J2000 = JD 2451545.0 = 2000-01-01 12:00 TT: $$ \...
Mike G's user avatar
  • 18.7k
6 votes

Calculating Earth-Sun distance only using trigonometry and terrestrial measurements

For the first hour and the last 75 minutes, it appears you neglected to take into account the direction of the Sun. As shown on this first image, you are measuring the Sun as if it were along the ...
Pierre Paquette's user avatar
6 votes
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Calculating Earth-Sun distance only using trigonometry and terrestrial measurements

This is an interesting experiment / project, and props for your significant effort toward executing it! The gist of your approach is trying to work out the distance using the solar altitude measured ...
geometrian's user avatar
5 votes
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Aligning the equinoxes to the cardinal points on a circular calendar

As Mike G said, the Sun's ecliptic longitude is ideal for this application. I see from your profile that you're a coder, with some knowledge of Python. Here's a short Python script that prints solar ...
PM 2Ring's user avatar
  • 15k
5 votes

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

The main factors that affect the length of the March equinox year (*1) over time can be divided into three components: Changes in the length of the solar day Changes in the length of the mean ...
Arcorann's user avatar
  • 151
5 votes
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Why is the time between equinoxes different?

The diagram below illustrates the reason, which is that the earth moves at different speeds during it revolution around the sun and the distribution of those speeds is not equal because the solstices/...
Tyler Durden's user avatar
5 votes

Why is the equinoxes only 2 days and not more?

Two further points to make are that: (1) because of the way we define sunrise and sunset as being when the entire Sun is below the horizon not just the centre of the Sun. (2) When the Sun is close ...
Science Geek's user avatar
5 votes

Why is the equinoxes only 2 days and not more?

The easiest graph to consider is the graph of day-length. If the day-length varies from 8 to 16 hours, there can only be two equinoxes: At the time of equinox, the day length changes at the fastest ...
predatflaps's user avatar
5 votes
Accepted

Is the claim that the star Sirius does not precess like other stars true?

Sirius precesses like the rest of the sky. Precession is caused by changes in the orientation of the Earth. Every star precesses as the Earth wobbles. Sirius also has a fairly high proper motion. But ...
James K's user avatar
  • 125k
4 votes

Sunrise and sunset times when compared between the vernal and autumnal equinoxes

Instead of looking at sunset/sunrise times, it's easier to look at noon, which is right in the middle between sunset and sunrise: 11:50am in March and 11:35am in September. The fact that noon doesn't ...
Glorfindel's user avatar
  • 4,800
4 votes
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Is the vernal equinox always in zenith somewhere on the equator?

The Vernal Equinox is defined by the point where the sun's path across the sky, the Ecliptic, crosses the Celestial Equator. It is always going to be on the Celestial Equator. Given the various ways ...
notovny's user avatar
  • 4,788
4 votes

Aligning the equinoxes to the cardinal points on a circular calendar

What you need is the Sun's ecliptic longitude for a given date. As you expect, it increases faster near perihelion (1.02°/day in early January) and slower near aphelion (0.95°/day in early July). It ...
Mike G's user avatar
  • 18.7k
4 votes

Accuracy of calculating the vernal equinox?

Calculating the exact time of the vernal equinox is essential for many astronomical calculations. I dispute that claim. What is true is that calculating the exact time of the vernal equinox is ...
David Hammen's user avatar
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4 votes
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In which town is the length of the day bigger on 23 of September?

That argument seems correct. A calculation by timeanddate.com would suggest that the day is about 9 seconds longer in Kazan than in Moscow on the 23rd of September, consistent with your prediction
James K's user avatar
  • 125k
4 votes
Accepted

At what time of the day of autumnal equinox the Sun's equatorial coordinates reach max value?

As noted in comments, at the september equinox, the sun's declination is decreasing (slightly) during the day, so the declination will be slightly higher in the morning than in the evening. But the ...
James K's user avatar
  • 125k
4 votes

Earth's rotation axis perpendicular to Sun-Earth line during equinox

During one solstice, the angle between the earth's axis and the line connecting the center of the Earth to the center of the Sun is as acute as it can get. Six months later, the angle is as oblique as ...
Darth Pseudonym's user avatar
3 votes
Accepted

What direction does 0° in the J2000 epoch point towards on January 1, 2000?

Or is 0 degrees the direction towards the ascending node 'equinox'? Which is NOT towards the Sun on January first... The answer is yes, more or less. From JPL's HORIZONS, the location of the Sun at ...
David Hammen's user avatar
  • 34.1k
3 votes

Aligning the equinoxes to the cardinal points on a circular calendar

The fact that equinox and solstice dates don’t line up with the quadrants of the circle is due to the ellipticity and eccentricity of Earth’s orbit around the Sun. Let’s go back in history a little. ...
Pierre Paquette's user avatar
3 votes
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Is this render of a ringed planet's shadow accurate?

This might happen if you model the ring as opaque but of very little thickness and in the same plane as the vector between the planet and the sun Seen edge on from the equator, the ring would block ...
antlersoft's user avatar
  • 3,455
3 votes
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Are the stars seen at the equator different throughout the year?

Yes, the constellations that are visible at midnight on Jan 1 are different than those visible at midnight on July 1 (6 months later). The website Stelvision has a star chart that you can customize ...
JohnHoltz's user avatar
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