How to Use Star Charts, Planispheres and Star Hopping

Tips on how to use star charts, finding objects using celestial coordinates, angular distance, what are arc minutes and arc seconds?

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. This is not as easy to understand as most coordinate systems, so let's have a closer look at what all these numbers mean.

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.

How to Use 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 is 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.

How to use Planispheres for Finding Stars and Constellations

Planispheres are simply two cardboard or plastic discs fastened together with a central pin, the upper disc has a large hole cut out so that you may see the star chart on the lower disc. The two discs are free to rotate around the central pin. The edge represents the horizon with appropriate north-south, east and west markings and everything within it is the visible night sky. Both discs will have date and time markings and when aligned with the day of observations, the view you see through the hole in the upper disc will be what you see in the night sky.

Despite the simple design, planispheres allow you to determine what bright stars are in the night sky on any date and at any time throughout the year.

How to use a Planisphere

Step 1 - Know Your Compass Points

Before using a planisphere, you need to identify and be familiar with the compass points from where you live. If you don't have a compass, use the Sun. It rises in the southeast and sets in the southwest in January.

Step 2 - Set The Planisphere

The two discs have a date and time markings. These need to be aligned by spinning the upper disc. For example, if you were to head out at 11 pm on 15th July you would align the 11 pm marker on the upper disk with the 15 July marker on the lower disc. The stars in the oval should now match those in the skies above.

Step 3 - Hold Up the Planisphere

Face North and hold the planisphere so that the word North is at the bottom. Now raise the planisphere above your head so that the sky chart remains visible to you and that North points to North and South behind you. You should now notice that the constellations in the sky match those of the star chart. If you change direction, simply rotate the planisphere in your hand around so that the corresponding compass points align.

Why can't I use a planisphere to find the planets or Moon?

Planispheres show objects that are fixed in the night sky relative to Earth - that's why they can be used year after year. However, this means that they can't predict the location of planets or the moon. Some manufacturers overcome this by printing details of planetary locations for several years on the back, but there is also a line printed on the chart itself which can help. The ecliptic often shown as a dotted line, marks the plane of the solar system, in which most of the planets orbit the sun. If you discover a star in the sky that's not shown on the planisphere then it's probably a planet.

Measuring Angular Distance

Angular distance is a measure of the apparent separation between two points from the perspective of the observer. Distances between objects in the night sky are measured in angles using degrees of arc, a bit like the angles of latitudes and longitude on the Earth's globe. One degree is equal to 1/360th of a circle.

Hands and fingers are very helpful for getting to grips with sizes within 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 need to find a dim object within the sky and a star chart shows that it's about 15° in a certain direction from a brighter, known star, then you'll be able to use your outstretched hand as a ruler to measure off the gap on the night sky. Between the two Pointer stars of the Plough, for instance, the space is about 5°.

Watch the Sky Move

The night sky changes in appearance over time. The stars keep the same positions relative to each other however appear 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 three hundred and sixty-five days, leading to stars being in slightly different positions at the same time every night.

If you stand in a single spot at 9 pm and see a star emerging above a rooftop to the East, then glance once more at 10 pm, that star shall be 15° higher. Fourteen days later it will be close to that place at 9 pm because the sky will appear to have moved 1° west every night.

Observing this variation will provide you with a good sense of the movement of our planet and the way it affects our view of the heavens.