Celestial Coordinate Systems

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Celestial coordinates are used to locate celestial bodies on the celestial sphere as seen from an observer on Earth. They are analogous to latitudes and longitudes which we use to locate objects on the Earth’s surface. To explain celestial coordinate systems, I’ll need to first present a few key terms.

Celestial Sphere

When observed from the Earth’s surface, celestial bodies such as stars, planets, nebulae, etc. follow a path around our sky. If we were to map this path onto a spherical canvas with the Earth at its center, then that canvas is the Celestial Sphere.

Fig. 1

Geographic North and South Pole

The top most point on the celestial sphere as seen from the northern hemisphere is the Celestial North Pole or Geographic North Pole and the top most point as seen from the southern hemisphere is the Celestial South Pole or Geographic South Pole.

Zenith and Nadir

The point directly above an observer and located on the celestial sphere is called the Zenith. Conversely, the point directly opposite to the zenith and located on the celestial sphere is called the Nadir.

Fig. 2

Ecliptic Plane

The plane in which the Earth orbits around the Sun is called the Ecliptic Plane.

Fig. 3

Ecliptic North and South Pole

The intersection points on the celestial sphere of a line through the Earth and perpendicular to the ecliptic plane are called the Ecliptic North Pole and Ecliptic South Pole. The angle between geographic and ecliptic poles is about 23.5° which is the same as the equatorial tilt.

Fig. 4

Celestial Equator

Fig. 2 (unlike Fig. 1) accurately portrays the equatorial tilt with respect to the tilt of the ecliptic plane. This tilt is about 23.5°. The circle formed when the equator is mapped onto the celestial sphere is called the Celestial Equator.

Vernal and Autumnal Equinox

When observing from the north pole, the Earth revolves around the Sun in an anti-clockwise direction. Notice in Fig. 2 that there are two points where the celestial equator and ecliptic plane intersect. As Earth moves from the far point to the near point (from the reader’s perspective), the Sun is increasingly higher in the sky. The point when the Sun is highest in the sky and the Earth is farthest from the Sun is called the Summer Solstice. The Sun takes longer to reach the high point and then go back down. This results in the longest day of the year. Similarly, when the Earth moves from the near point to the far point, the Sun is increasingly lower in the sky. The point when the Sun is lowest in the sky and the Earth is farthest from the Sun is called the Winter Solstice. The Sun takes lesser time to reach the high point and then go back down. This results in the shortest day of the year.

The near and far points are called Equinoxes because day time and night time are equal in duration. Vernal means Spring and consequently, the Vernal Equinox marks the start of spring. Conversely, Autumnal Equinox marks the beginning of autumn. The equinoxes are the only points when the Sun exactly rises in the east and sets in the west.

Galactic Plane

The plane on which the majority of the galaxy’s mass lies is called the Galactic Plane. The Galactic Poles of a galaxy are located perpendicular to its galactic plane (analogous to ecliptic poles). Even though the Milky Way Galaxy has a defined spiral form, not all stars are co-planar. So, the galactic plane is approximated.


Fig. 5

The galactic plane makes about 60.2° with the ecliptic plane.

Fig. 6

Precession

A spinning top is a great example to explain precession. In the absence of an external force, a spinning top continues to rotate around it’s axis which is perpendicular to the surface on which the top is placed on. If it is slightly nudged, the top begins gyrating around the axis forming a cone structure. This motion is called Precession.

Fig. 7

The Earth completes one full gyration around the axis of its cone structure in 26,000 years. Precession is caused due to the gravitational pull of other celestial bodies (primarily, the Sun and the Moon) on the Earth’s bulging equator. The Earth is not a perfect sphere. It is slightly bulged around the equator from the centrifugal force caused due to Earth’s rotation. Currently, the Earth’s axis points in the vicinity of the star, Polaris (or the Pole star). In a few thousand years, it will point to another star and the Pole star will no longer be Polaris.

Celestial Coordinate Systems

Horizon System

The Horizon Coordinate system relies on Altitude and Azimuth to locate a celestial body. The Celestial Horizon plane of an observer is perpendicular to the direction of gravity acting on the observer. The altitude of a celestial body is the angle formed between the celestial horizon plane and the line (say, L) connecting the observer and the celestial body. The azimuth is the angle as measured from True North (aka geographic north) eastwards towards the projection of the line, L on the celestial horizon.

Fig. 8

The horizon coordinate system has its limitations:

  • It does not work at the poles. The azimuth cannot be calculated when the observer is at the pole.
  • An observer’s horizon (and thus, the horizon plane) depends on their latitude and longitude location on Earth. Celestial bodies are also constantly in motion. This makes the horizon system coordinates very specific to time and location of the observer.

Ecliptic Coordinate System

The position of the ecliptic plane does vary, but very slowly. For our purposes, it can be considered invariant. The Ecliptic Coordinate system relies on locating a celestial body using the ecliptic plane and vernal equinox. The Celestial Latitude, β of a celestial body is the angle formed between the ecliptic plane and the line (say, L) connecting the observer and the celestial body. The Celestial Longitude, γ is the angle as measured from the vernal equinox eastwards towards the projection of the line, L on the ecliptic plane. Due to precession, the position of the vernal equinox keeps changing. This results in the ecliptic longitude of a given celestial body to increase by 1.396° per century.

Fig. 9

Unlike horizon coordinates, the ecliptic coordinates are not heavily dependent on the observer’s time and location on Earth. I don’t know why the ecliptic coordinate system went out of fashion but it was replaced with the equatorial coordinate system.

Equatorial Coordinate System

The Equatorial Coordinate system is based on Declination and Right Ascension. Declination is analogous to latitude and right ascension to longitude. While longitudes are based on the Prime Meridian, right ascension is based on the vernal equinox. The right ascension line passing through the vernal equinox is 0h and increases eastwards. They are apart from each other by 1 hour, where 1 hour is equal to 60 minutes and 1 minute is equal to 60 seconds. Declination is based on the celestial equator, so any points north of the equator are positive angles and those to the south are negative angles. Angles are measured in degrees, where 1 degree is equal to 60 arc minutes () and 1 arc minute is equal to 60 arc seconds (). 1” of latitude on Earth is about 101 feet.

Fig. 10

The equatorial coordinate system is independent of the observer’s location and time. However, both declination and right ascension are constantly changing (albeit, slowly) due to Earth’s precession. So, it is important to provide the epoch time along with the equatorial coordinates. This change in coordinates is insignificant considering human lifetimes.

Galactic Coordinate System

The Galactic Coordinate system is based on the Galactic Equator and the Galactic Center in the constellation, Sagittarius. The sphere encloses the galaxy and is centered on the Sun. Any celestial body north of the galactic equator is measured in positive degree angles, and those to the south are measured in negative degree angles. These angles form the Galactic Latitude (denoted by symbol, b) which is analogous to declination in the equatorial coordinate system. Galactic Longitude (denoted by symbol, l) is measured in degrees, eastwards from the galactic center. This is analogous to right ascension in the equatorial coordinate system. The North Galactic Pole is considered to be in the constellation Coma Berenices, and the South Galactic Pole in the constellation Sculptor.

Fig. 11

The galactic coordinate system is useful for observing how objects are distributed with respect to the galactic equator or plane.

Thanks for reading!

In this article, I looked at four different types of celestial coordinate systems. These are primarily used for locating celestial bodies in the sky to point telescopes in the right direction. Having said that, modern telescopes have computerized motors that can accurately point towards the celestial body when given the coordinates. In case you don’t have such a telescope, then knowing the coordinate system is vital.

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