For thousands of years, man has used the sky to help him find his way, tell the change of season and the time of day. But to the untrained eye, the sky can seem a random, confusing place. In fact, the stars and planets above us can be mapped in much the same way as we map landmarks and places on earth. Throughout time, man has recognised patterns in the sky by relating different stars to one another in groups or clusters that we call constellations. These patterns not only help us to remember certain stars but also to find them again with relative ease among the many thousands of others in the sky that together form the galaxies. Constellations are often recorded in the form of star maps, also called star charts or star atlases. In the same way that land maps help us find or place landmarks on earth by placing them in countries, cities and towns, star charts can help us place stars as well as other objects in the sky by describing their position within certain constellations or regions.
What are stars? Put simply, a star is a bright, glowing ball of gas. The energy of a star, created by a complex process of nuclear fusion, causes the star to radiate light that enables us to see it from the earth. Stars are often said to ‘twinkle’ in the sky, but this effect is in fact due to turbulence in the earth’s atmosphere and interstellar space that causes the light of the star to flicker. While the energy of stars can vary enormously, their brightness can also be an indication of their distance from earth. The light of a star can take many years to reach us, even travelling at the tremendous speed of light (300,000 km per second), since most stars are very far away. Usually, astronomers use light-years as a measure to describe distances in the sky that are too large for us to write down in conventional measuring units. A light-year is the distance that light can travel in one year, and is just a little less that ten trillion kilometres. Using light-years we can calculate just how far away different stars are from the earth. Even the stars closest to us can be more than 75 light years away. On nights with good visibility, more distant stars in the galaxies become visible and those in one galaxy appear grouped together in a hazy band through the sky referred to as the Milky Way.
Grouping stars into recognisable patterns, or constellations, does not mean that stars of a given constellation are grouped together at the same approximate distance. Most of the time, these stars are at very different distances from one another but just appear together by superposition on the sky. Constellations help us to remember the position of stars in relation to one another as we view the sky, but they also enable us to use them for navigation. For example, we can always find north by identifying the north pole, sometimes called the pole-star or the north-star which is aligned in the sky with the northern end of the earth’s rotational axis. This is done by first locating the constellation known as the Big or Northern Dipper (sometimes also called The Great Bear), which looks like a giant ladle or a chariot. The North Pole is aligned with the two stars that form the back of the chariot (see diagram below). By following the direction in which these stars point, and measuring a distance upwards of five times their separation, we end at the Pole star, as seen here:
The Big Dipper is a constellation common to many different cultures but it is important to note that most constellations are not universal. Over time, different cultures have recognised many different patterns in the sky, and identified their own constellations accordingly. As a result Chinese constellations are largely quite different to those we might recognise from western star maps. Likewise, different cultures have developed very different mythologies or beliefs around the stars and planets, a tradition better known as astrology. While in many cultures, astronomy and astrology were intricately linked for many years, we now recognise astronomy as a purely scientific discipline, entirely separate to astrology.
Planets are large spherical celestial bodies that orbit the sun. Planets do not radiate light in the same way as stars but they do reflect the light of the sun, which is how we are able to see them. The planets, including the earth and the moon, are more or less visible at different times, and while some can be seen by the naked eye at certain times, others are visible only through a telescope.
Diagram showing the approximate relative sizes of the planets Mercury, Venus, Earth and Mars from left to right. Distances between these planets are not shown to scale. Source image is from NASA's Solar System Exploration multimedia gallery.
Aside from stars and planets, comets are the largest objects in our solar system, measuring up to ten miles in diameter with tails that can extend millions of miles into space.
An image of Halley's Comet photographed by Yerkes Observatory in April 1910. Source image is from The New York Times which first published the photograph on 3rd July 1910.
Comets are objects made of solid particles and frozen gases that orbit the sun like tiny planets. Comets have a solid centre or nucleus made up of frozen rock particles which melt and turn to gas as the comet moves closer to the sun. This gas, and the particles which are released from the nucleus as it thaws, form the hazy tail that trails behind the comet. Comets can only be seen from earth at certain times in their orbit but because of records made by astronomers throughout history we are able to predict their return with a certain degree of accuracy.
The discipline of astronomy is concerned with the study of all celestial objects, their behaviour, and any events that occur outside of the earth’s atmosphere. Astronomers study these things in order to understand the workings and origins of the universe. It is a quest that has preoccupied people all over the world from ancient times until the present day and historical evidence of events in the sky is still proving useful for modern astronomers in this quest.