Constellations, Asterisms and Naked-Eye Objects

The naked-eye night sky

On a clear, dark night you can see a surprising amount without any telescope - the Moon, the planets, thousands of stars, the glow of the Milky Way, and if you are lucky a meteor or the aurora. This module is about recognising what you see: the natural objects, the human-made ones, and the star patterns we use to find our way around - the constellations and asterisms.

Star-gazing terms

Five terms carry the topic. Learn them before the checks lean on them.

Constellation or asterism?

The Plough (also called the Big Dipper) is a familiar pattern of seven bright stars. What is it, strictly speaking?

• An asterism - an informal pattern, part of the constellation Ursa Major
• A constellation in its own right
• A star cluster of thousands of stars
• A single double star

Constellation vs asterism

These two words are often muddled. The difference is whether the pattern is official.

Match each pattern to its description

Natural or made by us?

Select the THREE things in the night sky that are NATURAL, not made by humans.

• A meteor streaking across the sky
• The aurora (Northern or Southern Lights)
• The faint band of the Milky Way
• A satellite crossing the sky
• An aircraft with flashing lights

Seeing in the dark

Two simple techniques let you see far more. Dark adaptation: your eyes take about 20 minutes to become fully sensitive to the dark, so avoid bright white light - use a dim red light instead. Averted vision: to see a faint object like a nebula, look slightly to one side of it, because the edges of your retina are more sensitive to dim light than the centre.

Look to the side

Why can you often see a very faint star or nebula better by looking slightly TO THE SIDE of it?

• The edge of the retina is more sensitive to faint light than the centre
• The object is actually moving across the sky
• Looking away makes your eyes water and magnify it
• It removes light pollution from the view

Signposts in the sky

Asterisms are not just pretty - they are signposts. The two stars at the end of the Plough's "bowl" are the pointer stars: follow the line they make and you arrive at Polaris, the Pole Star, which sits almost due north. That is why Polaris is so useful: it barely moves all night, it shows you which way is north, and its height above the horizon is roughly equal to your latitude. Follow the curve of the Plough's handle the other way and you "arc to Arcturus", a bright star in Bootes.

Match each object to how you spot it

Find the Pole Star

Put the steps of finding Polaris on a clear night into order.

• Let your eyes dark-adapt for about 20 minutes
• Find the Plough in the northern sky
• Identify its two pointer stars at the end of the bowl
• Follow the line of the pointers away from the bowl
• Arrive at Polaris - it marks north, and its altitude is about your latitude

Name what you see

Three things you might see on a clear night. Identify each one.

• A steady point of light moves right across the sky in about four minutes, then fades.
• You want to see a faint nebula that vanishes when you stare straight at it.
• You need to find which way is north, with no compass.

Constellations and asterisms

An official sky pattern is a _____, while an informal one, like the Plough, is an _____. The Plough's pointer stars lead to _____, the Pole Star, whose altitude is about equal to your _____. To see faint objects, let your eyes dark-adapt and use _____ vision, looking slightly to one side.

Find Polaris, spot the satellite

On a clear night, explain how you would find Polaris and use it, and how you would tell a satellite apart from a meteor.

• Describe how to find Polaris using the Plough's pointer stars
• Say what Polaris tells you - direction, and how its altitude relates to latitude
• Explain how a satellite looks and moves differently from a meteor
• Mention one technique (dark adaptation or averted vision) that helps you observe