# What are the coordinates of an object in the sky?

Every object in the sky, like cities, has two numbers that define its location: right ascension and declination, often known as the object's celestial coordinates. Declination correlates to latitude, while right ascension corresponds to longitude. Objects can only be located within a certain distance on the surface of the Earth - the horizon - so they cannot be found beyond this limit.

Right ascension and declination are measured in degrees from zero at midnight on January 1st. Right ascension is measured clockwise from north (0° is north), while declination is measured east-west. So the coordinates of Venus are 6°54'43.0" North and 4°44'40.4" West, and those of Uranus are 84°24'6.8" North and 45°03'20.3" West.

The coordinates of a star can be determined using any modern telescope with an accurate clock and compass. First, locate the star in the sky. Then use the telescope to see how far away it is. Divide the angle between you and the star by the apparent size of the star in your telescope, then multiply this number by 60 to get the degree measurement for its right ascension. Multiply this value by 60 again for its declination.

## What are the celestial coordinates that describe a star’s position?

The position of an item on Earth is denoted by two coordinates: latitude (N-S) and longitude (E-W). Similarly, a star's location on the Celestial Sphere (the way the sky looks to us from Earth) is denoted by declination (abbreviated dec) and right ascension (abbreviated asc). The term "coordinate" means "a way to denote a place," and these two quantities are called coordinate stars. The constellation in which a particular star lies is often helpful in locating it.

For example, a star that lies near the southern horizon might be able to be located by observing where it is in relation to the constellations Orion, Virgo, and Leo. A star that lies directly overhead at mid-northern latitudes would have the same apparent location for all observers around the world. Thus, its celestial coordinates are exactly 0 degrees dec and 12 hours RA.

Celestial coordinates are usually specified as decimal numbers between -90 and +90 for dec and between 0 and 354 for RA (or 15 for eastern and 22.5 for western stars).

Stars can also be identified by their names. In the night sky, there are many thousands of stars that are well known to astronomers because they have been studied in detail over many years or even centuries. Some examples include Deneb, Altair, Vega, Arcturus, and Procyon.

## How do astronomers locate objects in the sky?

The equatorial coordinate system, which is based on the projection of the Earth's equator onto the celestial sphere, is used to list the coordinates of objects in the sky. In this approach, an object's location is described in terms of right ascension (a) and declination (d). Astronomers use a gnomon (an instrument used for measuring angles) to measure these quantities for themselves and for other astronomers who send them measured values.

Astronomers use telescopes to see objects in the night sky. The best known type of telescope is the reflecting telescope, which uses a mirror instead of glass lenses to gather light from objects behind it. Modern reflecting telescopes are highly engineered instruments that can be as large as several stories high with components that are smaller than life size. They are used for astronomy in all parts of the spectrum from gamma rays to invisible X-rays with wavelengths from about 100 nanometers (nm) to 10 microns. Reflected light from objects within the telescope's field of view is directed to photoelectric detectors called photomultipliers or pixels. These electronic devices convert incident light into electrical signals that are transmitted to computers where they are analyzed. Modern reflecting telescopes are capable of focusing on targets many times larger than what humans can see with our own eyes - even with the naked eye - providing access to faraway galaxies beyond the Milky Way.

##### Taylor Boyd

Taylor Boyd is an educator who has been teaching for over 10 years. He enjoys teaching because it allows him to use his knowledge and skills in a way that benefits others. Taylor loves nothing more than seeing the light bulb go off in a student’s head when they finally understand something.

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