Right ascension and declination

Hi,
in the last post I explained what is celestial equator and what is ecliptic, check it out if you did not read it.

Today I want to look upon right ascension and declination which is the system that lets you describe where stuff is on the sky and it does not matter where you are on Earth.


Basicly the gif on the left explains it: you have Earth inside your celestial sphere. Through the middle comes the equator, that makes sense but since Earth is tilted by 23° degrees to its ecliptic, in our picture it must be ecliptic that is tilted (it is always just a reference frame).

Of course there are two points where equator and ecliptic meet, as I said in the last post, those are autumn and spring equinox.

As you go around the circle your “value” gets higher, it is called right ascension (or RA). Full circle has 24 hours and when you take some place there it tells you in how much time the equinox that has “0 hours, 0 minutes” will get there.

The value of right ascension increases from west to east (’cause rotation).

Of course since we are in 2nd dimension we have to use another coordinate that will tell you how above you are from the equator (or below). This is called declination and is normally measured in degrees. Since furthest you can be from the equator are poles the angle wont be bigger than 90°.

Dragallur

Where do equinox come from?

Hi,
today I was trying hard to understand basics behind the coordinates that are used for stars and other celestial objects, once you grasp some basics you will get it, important part is the equinox and the “mechanics” that are behind it.

The celestial sphere with all stars at one distance


Basicly what you see on the left gif is the celestial sphere around the Earth (most of these things are geocentric since it is easier). Red line is the ecliptic. This is basicly the line that Sun follows over year, of course such motion is purely apparent but it is important to remember this one.

The white-green lines are the lines of equatorial coordinate system. The middle should be Earth’s equator though it does not match it much.

There are two points in which ecliptic and equator intersect. These points are called spring

How ecliptic is made (not to scale)

equinox and autumn equinox. When Sun passes through them the day is everywhere on whole Earth the same as the night. You already know that this happens happens twice a year, once for each equinox.

I will get to the coordinates in another post, hope you enjoyed this one 😉

Dragallur