Equatorial Coordinate system

The preferred coordinate system for astronomers around the globe, the equatorial coordinate system is not like the alt-az coordinate syste in the sense that it is not location dependant and can be used by anyone anywhere. You can think of it as projecting the Earth’s longitudes and latitudes onto the sky.

But similar to the alt-az coordinate system, the equatorial coordinate system also uses two measurements to locate an object in the sky. They’re the right ascension and declination, as can be seen in the image below (Credits: Julie Patris).

Right Ascension

Usually denoted by $\alpha$, the right ascension (abbreviated RA) is similar to the longitude on Earth. It is measured in hours, minutes and seconds. The right ascension is measured from the vernal equinox and increases eastward. It ranges from 0 to 24 hours. The right ascension is also known as the hour angle. The right ascension of an object is the time it takes for the object to cross the meridian from the vernal equinox.

Declination

Usually denoted by $\delta$, the declination (abbreviated DEC) is similar to the latitude on Earth. It is measured in degrees, minutes and seconds. The declination is measured from the celestial equator and increases northward. It ranges from -90 to +90 degrees.

If you’re at the geographic north pole of the earth, your coordinates will be 90 degree North. Your longitude is not defined and consequently doesn’t matter. Similarly, the pole star has a declination of 90 degrees and an undefined right ascension.

Vernal Equinox

Usually denoted by $\gamma$, the vernal equinox is the point on the celestial equator where the Sun crosses from the southern to the northern hemisphere. It is the point where the Sun is directly above the Earth’s equator. In the northern hemisphere, it is known as the March equinox and marks the beginning of spring. It is also the time when days start becoming longer than nights.

If you’re starting thinking about your even-semester end semester examinations, there is a good chance that the vernal equinox has already passed.

Equator

Imagine the Earth’s equator projected onto the sky. This is the celestial equator. It is the line that divides the sky into two hemispheres, the northern and southern hemispheres. The distance around the celestial equator is equal to 24 hours.

The declination of the equator is 0 degrees, 0 minutes, 0 seconds.

Poles

Similarly, projecting the North and South geographic poles onto the sky give us the celestial North and South pole.

If you’re at the geographic north / south pole, the celestial north / south pole will always appear at zenith.

Precession

Earth’s axis is not fixed in space; instead, it undergoes a circular motion over a period of approximately 26,000 years. This motion is primarily caused by the gravitational influences of the Sun and the Moon on the Earth’s equatorial bulge.

Precession has observable effects on astronomical phenomena. For instance, it influences the right ascension and declination of any given object and changes them slowly over time, including the position of the celestial poles and the equator and also affecting the locations of stars in the sky over long periods.

Precession is the reason that the pole star is not actually smack dab at the north celestial pole. It is also the reason that Vega will be at the north celestial pole and considered the pole star around the year 13,727 i.e. 11-12 thousand years from now.

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