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Published 15th July 2015 by

How to find objects in the sky using celestial coordinates, what are arc minutes and arc seconds? How big is the Plough compared to the Square of Pegasus?
Observational Astronomy Series
  1. A Beginner's Guide To Stargazing
  2. Dark Eye Adaption - How We See In the Dark
  3. Light Pollution
  4. Using Star Charts and Measuring Distance
  5. Top Tips for Binocular Astronomy
  6. Moon Watching - How to Observe the Moon
  7. Buying Your First Telescope
  8. Your First Night With Your First Telescope
  9. Sky Orientation through a Telescope
  10. Polar Alignment of an Equatorial Telescope Mount
  11. All About Telescope Eyepieces
  12. Useful Astronomy Filters for Astrophotography
  13. How to Photograph Constellations

If you are looking for a particular object in the sky chances are you have a set of celestial coordinates measured in Right Ascension and Declination.

For this example, we'll use the star Deneb which can be located using the coordinates 20h 41m 25.9s Right Ascension, +45° 16' 49" Declination. Clearly this is not as easy to understand as most coordinate systems, so let's have a closer look at what all these numbers mean.

The Celestial Sphere

The Celestial Sphere

Right Ascension, measured in time, is the projection of longitude onto the celestial sphere. Zero hours starts at the First point of Ares (the point in the sky at which the Celestial Meridian, the Celestial Equator and the Ecliptic meet) and measures the full 360° of the celestial sphere. One hour Right Ascension describes the movement of the sky due to the Earth's spin over an hour and is equal to 15° (15° * 24 hours = 360°). As with time, an hour of Right Ascension is divided into minutes which are also divided into seconds. These all likewise describe the movement of the sky over the specified time.

Declination is akin to the measurement of latitude projected into the sky. Zero degrees of declination represent the celestial equator (a projection of Earth's equator) with +90° representing the North Pole and -90° declination the South Pole. A single degree of declination is further divided into 60 arcminutes, and each arcminute into arcseconds for greater precision.

Using Star Charts

With the help of star charts, you'll be able to find a multitude of objects under a dark sky. It may look confusing at first, with the directions being backwards - West is on the right and East on the left. Unlike most maps, star charts show what is above you, not beneath you. If you hold the chart above your head with North pointing North you'll find East and West are pointing correctly.

To use it to look at things in different directions, hold it so that the bottom edge corresponds to the compass direction you are facing. It will then show the sky as it looks from that horizon and on up over your head. If you don't know which direction is which, use the Plough to find Polaris. Once you find North, the rest will follow. It's best to use a red led torch to see the chart in the dark - that way it won't ruin your night vision.

Using Star Charts

Using Star Charts

Measuring Distance

Distances between objects in the night's sky are measured in angles using degrees of arc, a bit like the angles of latitudes and longitude on the Earths globe. One degree is equal to 1/360th of a circle.

Hands and fingers are very useful for getting to grips with sizes in the sky. Held out at arm's length the width of your little finger is around 1° and the width of your thumb about 2°. Three fingers are about 5° and the width of your fist is around 10°.

If you want to find a dim object in the sky and a star chart shows that it is about 15° in a certain direction from a brighter, known star, then you can use your outstretched hand as a ruler to measure off the distance on the night sky. Between the two Pointer stars of the Plough, for example, the distance is about 5°.

Your hand can become a useful measuring stick on the night sky when held out at arms length

Your hand can become a useful measuring stick on the night sky when held out at arms length

Watch the Sky Move

The night sky changes in appearance over time. The stars keep the same positions relative to one another but seem to move as a whole. This is because the Earth spins on its axis every 24 hours, so the globe revolves 15° to the west every hour. The Earth orbits the Sun in about 365 days, resulting in stars being in slightly different positions at the same time each night.

If you stand in one spot at 9 pm and see a star rising above a rooftop to the East, then look again at 10 pm, that star will be 15° higher. Fourteen days later it would be close to that position at 9 pm as the sky will seem to have moved 1° west each night.

Observing this change will give you a good sense of the motion of our planet and how it affects our view of the heavens.

Tutorial Series

This post is part of the series Observational Astronomy. Use the links below to advance to the next tutorial in the couse, or go back and see the previous in the tutorial series.

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