The Night Sky (Coordinate Systems)

Now, we know how to calculate the distances of the Stars, Galaxies, etc. from the Earth. So, now we are going to see how to specify the exact location of any object which we see in the Night Sky. This is because, if we are observing any Star or witnessing any event and to report it, we need to specify the exact coordinates. We can’t just simply say LOOK UP or LOOK THERE.

On Earth, we have a coordinate system too, RIGHT! We use this system to specify the position (location) of any object present on the Earth. The coordinate system on Earth is based on Latitudes and Longitudes.

For Example-

Coordinates of India = 20.5937° N, 78.9629° E.

Coordinates of Himalayas = 28.5983° N, 83.9311° E.

Similarly, we have coordinate systems for measuring the exact positions of the objects in the Sky as well. Before we proceed any further with it, let us first see a few basic things which will make it easier to understand the coordinate systems of Sky.

Latitudes-

Latitudes are the imaginary lines drawn on Earth which run parallel to the Equator. These lines start from Equator and end at Poles. There are a total of 13 latitudes, 6 on each side of Equator and Equator itself. Equator is considered to be of 0° latitude. As the Equator and Poles are perpendicular to each other, the region between Equator and Poles is divided into 6 equal parts on each side, each latitude is of 15° (15*6=90). They specify the North-South location on Earth’s surface.

The latitudes between the Equator and the North Pole are marked as +15°, +30°, …., +90° or 15°N, 30°N, …., 90°N. The latitudes between Equator and South Pole are marked as -15°, -30°, …., -90° or 15°S, 30°S, …., 90°S.

Longitudes-

Longitudes are the imaginary lines which join both the poles. These are the converging lines that start from North Pole and end at South Pole. They are used to specify the East-West location on the Earth’s surface. It’s basically an angular measurement from the Prime Meridian, which is considered as 0° longitude. (Meridian: The line joining the points of the same longitude).

We have a total 24 longitudes, in which 1 is Prime Meridian, 1 is shared by both Eastern and Western longitude, 11 each are Eastern and Western longitudes. So now, we have 24 divisions for longitudes and as Earth is spherical (360°) each longitude is of 15° (15*24=360). From each direction (East and West) their respective longitudes are up to 180° only. The East longitudes are marked as positive longitudes whereas West longitudes are marked as Negative longitudes.

The Eastern longitudes are marked as +15°, +30°,...., +180° or 15°E, 30°E,...., 180°E. The Western longitudes are marked as -15°, -30°, ...., -180° or 15°W, 30°W, ...., 180°W. The longitude 180°E and 180°W are the same or we can say shared longitude. 

Zenith and Nadir-

Zenith is the point directly overhead to an observer. It’s pretty easy to mark Zenith, we start by marking a line which joins Northern and Southern Poles (Celestial Meridian) and then another line which joins Eastern and Western Horizons. Now, we get a point of intersection, this point is directly overhead to an observer and is known as “ZENITH”. It’s also known as the highest point.

Nadir is the point directly beneath the observer. If we put a perpendicular from the Zenith to the surface and extend that perpendicular below that we get a point which is opposite to the Zenith and is directly beneath the observer. This point is known as “NADIR”. It’s also known as the lowest point.

Celestial Sphere-

This concept of Celestial Sphere is based on Geocentric Model, we all know that the Heliocentric model is the most accurate model, but sometimes considering things according to the Geocentric Model for small parts makes it easier to understand.

Let us assume an imaginary sphere having radius 5-6 times that of Earth and Earth placed at its center. All the latitudes and longitudes marked on Earth are projected on that sphere and all the Stars and objects we see in the Sky are embedded in it.

The Stars in the Celestial Sphere revolve around Earth from East to West. This is because the Earth rotates from West to East and in the Geocentric Model we assume Earth to be at rest, so we see Stars moving. 

Ecliptic-

Ecliptic is the path traced by the Sun in the Sky. Basically, as we all know that the Sun stays in a constellation for around 1 month and hence, 12 constellations in 12 months and if we mark each constellation and draw a line through them, we get the Ecliptic. Ecliptic can also be stated as the apparent path of the Sun in the Celestial Sphere.

Ecliptic is also referred as the plane of revolution of all the planets. So, best way to mark planets is to mark the Ecliptic and then search planets along it.

Now, we know these basic terms, so let’s move on to the “COORDINATE SYSTEMS”.

• Coordinate Systems- 

As we all know that, the Night Sky is perceived in 2D by our eyes. So, we need only 2 coordinates to mark any object. We have 2 types of systems which are widely used. They are-

1.   Altitude-Azimuth System (Alt-Az)

2.   Right Ascension-Declination (RA-Dec) / Equatorial System

• Altitude- Azimuth System:

In this system, one of the coordinates is provided by Altitude and the other one is provided by Azimuth. So, by using these coordinates we can mark any object. These coordinates change from area to area.

Azimuth provides the horizontal coordinate. It’s an angular measurement of the distance of any object. We measure starting from North moving towards East and coming back to North.

Altitude provides the vertical coordinate. It’s an angular measurement of the height of any object. We simply measure the angle made by an object with the horizon. If the object is directly in front of us then the angle is 0° (the object is horizon) and if the object is directly above then the angle is 90° (the object is at Zenith). 

• Right Ascension- Declination / Equatorial System:

In this system, one of the coordinates is provided by RA and the other is provided by Dec. This system is kind of similar to Latitude- Longitude system on Earth, as the Latitude is here replaced by the Declination and the Longitude here is replaced by the Right Ascension. The reason is that, as we have projected latitudes and longitudes on the Celestial sphere and the Earth rotates, so this means that these projections will move too and hence, not possible to use them as a coordinate system. In RA- Dec system, the coordinates of any object are same for all regions.

Right Ascension provides the horizontal coordinate. It is an angular measurement of the distance of any object and is measured moving Eastwards on the Celestial Equator from the point of Vernal Equinox. The point of Vernal Equinox is considered to be 0 hr RA. The Celestial Equator is divided into 24 equal parts, each of 1 hr. Each day lasts for 24 hr, so we make 24 divisions on the Celestial Equator, each division is of 1 hr, that means (1/24)th part of the Celestial Equator. Currently, the Vernal Equinox lies in the constellation of Pisces.

Declination provides the vertical coordinate. Declination is nothing but the latitude itself. It’s an angular measurement of the height of any object and is measured from the Celestial Equator. If any object is at the Celestial Equator then it will have Declination of 0°. The range of Declination is from -90° to +90°. The objects at Zenith and Nadir will have Declination of +90° and -90° respectively.