35 votes
Accepted

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
  • 1,791
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
4 votes
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Why doesn't the solar azimuth vary uniformly

The Sun's hour angle, measured around the celestial equator, changes at a nearly constant rate. The gnomon of a sundial is aligned with the polar axis to take advantage of this. The celestial equator ...
Mike G's user avatar
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3 votes

How to calculate the maximum and minimum solar azimuth at a given location?

Thanks to Ralf Kleberhoff for Pointing me in the right direction. Step 1: Find Summer Solstice Date For the accuracy needs of this question simply finding the hemisphere and picking the most common ...
antgiant's user avatar
  • 213
3 votes
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Error when calculating Alt/Az from Ra/Dec

I have the answer to your question, in case you have not already worked it out! You have used the wrong value for the Longitude of the Sun in your calculation of GMST0. You used the Ecliptic Longitude,...
John H's user avatar
  • 46
3 votes

Error when calculating Alt/Az from Ra/Dec

The algorithm on that page is very simplistic and is probably the source of a good portion of that error. It's best to test your code using a very accurate ephemeris, even if your final goal is to ...
Greg Miller's user avatar
  • 5,867
3 votes

an exercise from Spherical Astronomy by Smart

Constant for every star with different declination's. Though you would have to assume the the declination is such that it can have an azimuth of 90 each, not all stars will. Depending on your ...
James Screech's user avatar
3 votes

Azimuth angle and elevation angle of a heliostat mirror

As I said in my comment, you appear to be using an Azimuth system different from what most astronomy programs will return it in. As long as you're consistent, it won't matter, but double check to ...
Greg Miller's user avatar
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3 votes

Why does the altitude-azimuth mount have a blind spot near zenith?

The blind spot is a limitation on tracking an object through zenith, not on where the telescope can move to. An alt-azimuth telescope can rotate around two axes. If an object transits through the ...
ProfRob's user avatar
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2 votes

Why does the sun rise north of east between the vernal and autumnal equinox?

[For this explanation, it's helpful to ignore the slight apparent $1^\circ$ motion of the Sun throughout the day]. The way I like to think about this is imagine standing at the North Pole in the ...
Greg Miller's user avatar
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2 votes
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Determine sun azimuth and elevation for shading a window throughout the year

You can use a package such as the astropy or skyfield packages for python. It can calculate the position of any astronomical object. You can use a website like suncalc.org This lets you find the ...
James K's user avatar
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2 votes

Obtaining deltaT for use in software

The actual equations used by NASA are located here: https://eclipse.gsfc.nasa.gov/SEcat5/deltatpoly.html I failed to find any pre-written code and consequently wrote my own in Swift. The equations ...
Aloft's user avatar
  • 91
2 votes

Path of sun calculation using java

The NOAA formula sheet expresses some quantities in radians, some in degrees, and some in minutes of time. It does say which is which, but it's easy to miss. A few minor code changes can help expose ...
Mike G's user avatar
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2 votes
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How to calculate the maximum and minimum solar azimuth at a given location?

The azimuth angle from due south to the point where an object rises or sets is a function of the latitude (lat) and declination (decl), as follows: cos(angle) = -sin(decl)/cos(lat) This ignores ...
JohnHoltz's user avatar
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1 vote

How to calculate the local time of sunrise and azimuth of rising point of Sirius?

This is from: https://www.celestialprogramming.com/risesetalgorithm.html . It has a Javascript implementation and example you can run for the rise, set, and transit times. Keep in mind that if you ...
Greg Miller's user avatar
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1 vote
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How to calculate the local time of sunrise and azimuth of rising point of Sirius?

The solution is probably something like this: $s = t + a$, where $t$ is local hour angle and $a$ is right ascension. $t = \arccos(-tg \phi \times tg \delta)$ $t = \arccos(0) = 90 (=6^h), \ \ $ since $\...
ALiCe P.'s user avatar
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1 vote
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What is the relationship between the shape of the analemma and the sun's azimuth at sunset at a given latitude?

There are two reasons the real azimuth is different from the theoretical sine curve: The real azimuth is sinusoidal (like a sine curve) but not exactly a sine curve. More information is given below. ...
JohnHoltz's user avatar
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1 vote
Accepted

Tracking the Sun from Sunrise in the East to Sunset in the West

Conventionally, 0 is North, 90 is East, 180 is South and 270 is West; in both the Northern and Southern Hemisphere. These are directions. This is nothing to do with GPS, which is about location, not ...
James K's user avatar
  • 122k
1 vote
Accepted

Solar Azimuth angle Derivation

First we set up our global coordinate system as following: Origin: center of Earth. $\vec z$: earth's axis (North Pole is the positive direction), $\vec x$ and $\vec y$ represent the equatorial plane ...
Salmonella mayonnaise's user avatar
1 vote

Solar Azimuth angle Derivation

Have a look at https://www.pveducation.org, specifically section 2.4 on Terrestrial Solar Radiation. It describes the equations required and has some online calculators and interactive plots. If you ...
Emanuel V's user avatar
1 vote

Solar Azimuth angle Derivation

What I think you're trying to do is workable, but approximate. You first find the Sun's geographical position (GP), the point on the Earth where the Sun is in zenith at the time of the observation. ...
stretch's user avatar
  • 1,791
1 vote

Refinement to sun/wristwatch estimate of N/S direction

I wrote a test program to compute the deviation between the wristwatch estimation and the real sun position, based on a program by dabasler. The results quality depends on latitude, month and time of ...
Ralf Kleberhoff's user avatar
1 vote

Refinement to sun/wristwatch estimate of N/S direction

The method is generally reliable between 9 am and 3 pm. The other consideration is when one is using the method in either of the tropics: Cancer or Capricorn. It is generally accepted that in the ...
Fred's user avatar
  • 2,169
1 vote

Determining Horizontal Coordinate System data (azimuth, altitude) for geographic locations

This can be done using the Python package Skyfield fairly easily. Here is a shell of a script; I used some dictionaries to hold data, you may want to do something else. I've stored a lot of goodies ...
uhoh's user avatar
  • 30.7k
1 vote

How to calculate solar coordinates. (Right Ascension, Declination, and ultimately the Altitude and Azimuth of the Sun)

For the Sun at JD 2458599.125, RA = 32.8° and dec = 13.2° are correct regardless of geographic location. In the alt/az calculation, LHA should be 323.6°...
Mike G's user avatar
  • 18.7k
1 vote

How do I calculate the sun's azimuth based on zenith, hour angle, declination, and latitude?

I've confirmed that this formula is in fact correct. I did have the wrong units in the code, which @Glorfindel suggested: sun's hour angle was in hours instead of radians. I did need to adjust the ...
nnyby's user avatar
  • 111
1 vote
Accepted

Solar azimuth for celtic fire festivals

The azimuth of the sunrise (or sunset, or any object) is a function of the Sun's declination and observer's latitude. It can be calculated from the following forumla: $$\cos(\theta_R)=-\frac{\sin(...
JohnHoltz's user avatar
  • 8,001

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