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Celestial Coordinates

Mapping Objects in the Sky

"Where is it?" is a common question from those looking for stars, planets and other objects in the night sky. This page reviews the Celestial (or Equatorial) Coordinate system used by astronomers to map objects that they are studying.

Equatorial Coordinate System

The Celestial or Equatorial Coordinate System was developed to help astronomers describe where in the sky an object can be found. Imagine the Earth inside a sphere many times larger than Earth with all the stars placed on it. We know that some stars are closer than others but it is a way to make a map of the sky called the Celestial Sphere.

A line drawn through the Earth's poles marks the North and South Celestial Poles. The Earth's equator becomes the Equatorial plane of the Celestial Sphere. Objects like stars can be above or below this imaginary plane by up to 90° with plus degrees indicating above or towards the North and minus degrees indicating to the South. This is called Declination.

Imaginary lines can then be drawn around the Celestial Sphere from North to South every 15 degrees around the circle made by the equator. The first line or 0 Hour is drawn through a point where the sun crosses the celestial equator at the vernal equinox in March. It is also known as the Prime Meridian and the First Point of Aries, the Ram constellation.

Each line marks one hour called Right Ascension. It is measured in hours because a circle has 360° which when divided by 15 equals 24, the number of hours in a day. Parts of hours are designated by minutes and seconds. Even smaller sections are divided by "arc-seconds" because they measure just a tiny portion on the arc made by the circle of the equator.

This system is confusing at first because we are not used to having hours and minutes measuring distance on a circle rather than time. To further complicate things the astronomer has to know where he or she is in relation to 0 Hour.

Keeping all this in mind, we can say a star has so many degrees of Declination and so many Hours of Right Ascension and astronomers all around the world can point their telescopes to it when it can be seen from their location on the Earth.

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For more information on celestial coordinates, go to these pages:

NASA, Celestial Coordinate System
Cornell University, Celestial Sphere

Star Maps

Click for larger Star Map for September

Star maps or charts are used to find constellations, variable or double stars, nebulae and other objects. Round maps like the one at left of September are designed to show which constellations can be seen in a particular month. The gray area running across the map is the Milky Way. Planispheres can show the entire year by turning a printed disk inside marks showing time and month. If you hold the map above your head, point the side labelled North towards the north, you can pick out stars belonging to constellations. There won't be any connecting lines, of course, because they are there just as a guide.

Click here for larger Sky Map image.

Star maps can show thousands of stars and their brightness (magnitude) by making the dot representing a star larger (for brighter) or smaller as needed. Since many more stars can bee seen with a telescope or binoculars, such star charts help astronomers find just the star or object they are looking for.

Planets are usually placed on star maps in monthly publications like Sky & Telescope and Astronomy magazines because they are always moving into different constellations as they and the Earth orbit the sun.

You can check out individual star maps and see how they change each month or see all the maps on one page.

Common Astronomical Objects

This is a short list of objects you can see with just your own eyes and binoculars.

Common Astronomical Objects
Object Description
The Moon Craters, lava flows and mountains
Jupiter and Saturn Binoculars show moons of Jupiter, "ears" on Saturn
Milky Way The cloud of stars across the sky is the edge of our own galaxy
Constellations 65 can be seen from North America (about 40° latitude)
Big Dipper Asterism Part of the Constellation Ursa Major
Polaris The North Star is in the Little Dipper's handle
Double Stars The middle star of the Big Dipper's handle is really 4 stars
Orion Nebula Cloud of dust and gas in the sword of the Constellation Orion
Pleiades Star cluster near Orion
Andromeda Galaxy Neighbor galaxy like our own
Lagoon Nebula Cloud of dust and gas near the Constellation Sagitarius
M13 Star Cluster A ball of stars near the Constellation Hercules

Review the Observing Hints on the Basic Astronomy Facts page and look at your universe from your own backyard!

Click for Astronomical League website For more information on celestial objects, go to the Astronomical League pages describing their viewing programs. The site below will open up in a new window. Close the window to return to Kid's Cosmos. Click here to go to the  Astronomical League.

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© 2011 Kid's Cosmos
© 2011 Kid's Cosmos
Kid's Cosmos