Explaining the angles when calculation the position of the sun

Have you ever tried to understand all the terminology used when looking up things like Sidereal Time, and Right Ascension.
The webapplication I created, shows you exactly what is what. On this page a more verbal explanation is given, right along some things you can do in the app to make it more clear.

The webapplication can be found on https://solarsystem.bartje.org/solarsystem.php?apptype=gmst_explain

Wiki information can be found here : https://en.wikipedia.org/wiki/Equatorial_coordinate_system 


Equatorial Plane


Sun Pointer

Longitude Pointer

Sun Trail

Sun Distance Indicator

Angle Calculations

The application shows how you can envision the different angles that are used to calculate the position of the sun, earth, planets, and all other celestial objects.
The explanation here uses the sun as it's primary example. This is done intentionally as I found it hard to get used to the terminology when going through all the explanations on the internet. However where I use the sun in these descriptions, the method is valid for most other objects in the sky.


Animate Calculation

Greenwich Mean Sidereal Time

The Greenwich Mean Sidereal Time is the angle between the Vernal Equinox and the Prime Meridian, on the celestial plane.

The word mean describes that


LM Sidereal Time

The local mean sidereal time is the angle(or time) between the vernal equinox and the meridian of the observer, on the celestial plane.

Right Ascension

The right ascension measures the angle along the celestial equator from the vernal equinox to where the sun is at that moment.

To show how it works, pick on of the prime times: Vernal Equinox (0o), Summer Solstice(90o), Autumnal Equinox(180o) or Winter Solstice (270o).
When you move the date the angle will decrease when going back and increase when going forward in time. Note how the earth rotates almost precisely with the Right Ascension angle when going up (or down) 1 day.
When inceasing the time, the angle will change too, of course, but it is smaller: The earth does not move that much around the sun in 1 hour.

However it will not change when moving you location from east to west (or north to south): The value is fixed on the position of the earth around the sun.

Set the sequence function 1 to day, and see how it changes during the year, make sure to set the display option for Right Ascension (white) on, and leave the others off to not get confused.
You could leave the Sun Pointer and the Point of Aries (red) on too, it will make even more sense.


Hour Angle

The Hour angle measures the angle along the celestial equator from the observer's meridian to the hour circle passing through the sun. In layman language: It is the angle (or time) since noon on the observers place.

In the application there are two options, one from P.o.A. (Point of Aries). This is not the real situation, but lets you see how the angle changes over time. One noticeable thing is, to view


Universal Time

The Universal Time is the hour angle between the suns position and the Prime Meridian, only the not at 0o, but at the other side of the earth at a 180o.
So essentially it is the time since midnight in Greenwich.

In the app this is visible by always showing the angle from from the Prime Meridian.

To show same examples:

Set lat/lon to Greenwich (use Earth Icon  and select Greenwich) and click on the Sun Icon . Set display option to show Universal Time: The result is a semi circle. Now move the time to 0hrs. The earth will spin 180o and the purple arc is gone.



The longitude option shows the longitude from Greenwich, on the celestial equator.

Point Of Aries

The point of Aries is the place where the Equatorial Plane and the Celestial Plane intersect. Those are fancy words for the point where the sun is when spring begins (around march 22nd).