So you’ve bought your telescope or binoculars and you know how it works. You’ve done a bit reading on the subject of astronomy. It’s time to get out and do a bit of observing! But where to start? You have read about all those planets and stars out there, but how do know which are visible tonight? Although you may read about the subject it’s hard to get an understanding of the night sky without actually looking at it. And the problem is it’s changing all the time, with each passing hour and day. The sky at 8pm tomorrow will look slightly different from the same time tonight. Most star charts are no more than monthly, which can make it difficult for the astronomer starting out.
Well, first of all you need to get your bearings, meaning that you know where north and south are relative to your observing location. To get an idea the Sun moves across the sky on an approximate north-south line called the meridian. The Earth rotates from west to east, which is why the relative motion of the Sun is in the opposite direction. The other planets and stars also appear to have a similar motion, but in reality it’s not quite so simple.
Once you get used to observing the night sky you will realize that it rotates around a central celestial pole, which is directly above the Earth’s pole. This is an essential point to know when getting your bearings. It is located due north or south depending in which hemisphere you are located, at the same angle above the horizon as your latitude. So if you are at latitude of 35°, the celestial pole is 35° above the horizon. For those of you in the Northern Hemisphere, you are lucky, the bright star of Polaris, in the constellation of Ursa Minor, is near the north celestial pole. No such bright star pinpoints the position of the south celestial pole. The bright stars of Ursa Major (shaped and known as the Big Dipper or the Plough) are convenient signposts to the celestial poles.
The sky rotates anticlockwise around the celestial pole in the Northern Hemisphere, and clockwise in the Southern Hemisphere. This means that some parts of the sky are visible all night, all year round. Those stars which are currently between the pole and the horizon will be above the pole in 12 hours’ time, or at the same time of night in six months’ time. Therefore as you look towards the pole you will see the same stars whenever you observe. Only their orientation changes. This is the circumpolar sky.
As you would expect, half-way between the two celestial poles is the celestial equator, which is always above the Earth’s equator, and runs in a curve across the sky from due east to due west, reaching its greatest height on the meridian.
The ecliptic is the path the Sun travels through the sky throughout the year. The paths of the Moon and the planets are close to this line. The path of the ecliptic changes with the seasons. On a midsummer night the Sun is low in the sky, during the day it is high. The reverse is true on a midwinter night.
The Milky Way, the band of light composed of many stars towards the centre of our galaxy, appears as a great circle around the sky. Although too faint to be seen with the naked eye under city lights, it can be seen through a telescope. On September evenings the Milky Way lies virtually overhead in the Northern Hemisphere, with the dense star clouds of the constellations of Cygnus and Sagittarius clearly visible. On April evenings the Milky Way lies close to the northern and eastern horizons, and is only visible at very dark sites. At this time of year the sky is emptier of stars and nebulae, allowing many of the distant galaxies to be seen, such as the large cluster in Virgo. The brightest star group, Orion, dominates the winter night sky.
The way the stars move is hard for an astronomer starting out to understand and follow. You should read some practical astronomy books, and look at the monthly and seasonal star charts to see what happens to the constellations as the year progresses. One way of visualizing this is by use of a planisphere, which will be discussed in a later article.