Star Charts

demonstrate the use of a planisphere, star chart or computer software in order to plan an observing session
demonstrate an understanding of the terms ‘ecliptic’ and ‘zodiacal band’ on a star chart
use star data and charts to determine the time at which a star will cross the observer’s meridian.
explain the apparent east-west motion of the night sky
recall that stars cross the observer’s meridian and culminate when they are due south

People used to imagine that the stars were painted on a giant sphere which surrounded the Earth. Although we know that this is not the case it is still a useful model. How can we represent the celestial sphere on a 2 dimensional piece of paper?

Below is a screenshot from the excellent Stellarium planetarium software. (This is a free download. See the Links page) One can really imagine being inside a sphere. The blue line is the celestial equator. The red line is the ecliptic, the path of the Sun. Notice that the two line cross at a RA of 0h, on the celestial prime meridian.

If we put these grid lines on a flat piece of paper and plotted the RA (x axis) and Dec (y axis) of all the stars on it there would be quite a bit of distortion, especially near the poles.

Below is such a chart, at least showing most of the sky. I haven't included any stars but have put on the celestial equator and the ecliptic.

If we create charts for smaller parts of the sky, e.g. for single constellations, then much less distortion occurs and these charts are really useful for mapping the sky. Especially for constellations close to the equator.

To see charts for all the constellations the best website I know is at http://www.astrofilitrentini.it/mat/costell_eng.html

Here I have added the dates that the Sun is at different times in the sky throughout the year. The stars, of course, stay in the same place.

Important - The constellation of Orion is best viewed in the winter. Why?

Most of the stars in Orion have RA's between 5h and 6h. In June the Sun will pass very close to Orion so Orion (and the Sun) are in the sky during the day. In December the Sun is on the other side of the celestial sphere to Orion so it would be prominent in the night sky. Stars with RA's of about 18h would best be viewed in the summer.

Using a Planisphere

A planisphere consists of a star chart and horizon overlay. Depending on the time of year and the time of night we see different stars in the sky. To use the planisphere one faces North then turns the horizontal overlay until the month and time are correct. The stars which can be seen on the planisphere should match up with those in the sky.

The best way to learn to use one is not by reading stuff like this but by getting one and using it. In the UK they are available from Waterstones and W H Smiths.