Autumn Night Sky transcript

Autumn Night sky

When we gaze up at the night sky it appears to be a random collection of pinholes in the fabric of night. But look again and you see definite patterns begin to emerge; we refer to these patterns as constellations.

The ancient Greeks began to form patterns thousands of years ago. They would invent shapes amongst he stars and invent stories to go with these shapes.

Stories of forbidden love, stories of terrifying creatures and stories about magical animals. Most people can usually identify at least one constellation, often the Plough or Orion.

The plough, part of the constellation Uma, is known as a circumpolar constellation and for most people who live in the northern hemisphere the Plough can be seen all year round as it never sets below the horizon.

The constellation of Orion is different; we can only see Orion at certain times of the year, during winter Orion stands proud in the night sky but during Summer he is no where to be see. The stars that make up Orion haven’t really moved we’re just looking at the sky from a different position as we move from season to season on our path around the Sun.

For example, in July during summer, Orion is high in the sky but during daytime we can’t see him because of the Sun’s glare. Whereas when Orion is high in the sky in January its night-time.

Because of this we see a different sky each season and because it’s now autumn let’s take a look at the autumn night sky.

Let’s start off with a constellation most people can find, the Plough. Some people think the plough looks like a saucepan; here’s the curved handle and here’s the pan.

These two stars are known as ‘the Pointers’. If we draw an imaginary line this way we come to the North Star. This star always points in the direction north.

If we continue our imaginary line this way we come to the ‘W’ shaped constellation of Cassiopeia. Cassiopeia was a mythical queen of Ethiopia who boasted her daughter, Andromeda, was more beautiful than the sea-nymphs.

This feint fuzzy patch is the Andromeda galaxy, a collection of billions of stars at a distance of 2.2 million light-years away it is the farthest object we can see with the naked eye.

Because of the Queen’s boast, Poseidon, god of the sea became very angry and ordered that Andromeda should be sacrificed to the sea-monster Cetus. Otherwise the kingdom and all its people would be destroyed.

So Andromeda was chained to a rock by the sea to await the inevitable. At this same point a brave and handsome young man named Perseus came across this scene. Perseus had just proven his bravery by destroying the evil snake headed monster Medusa and was carrying her head back as a trophy. Medusa was such a horrible sight that anyone who set eyes on her was instantly turned to stone.

Follow our line this way and we come to the constellation of Perseus. An interesting feature of this constellation is this star, Algol – known as the demon star.

The light from Algol fades every 2.8 days and then brightens again 4 hours later. This is because Algol has a companion star that passes in front, partially obscuring its light. It’s called the ‘Demon Star’ because it is said to represent the pulsating eye of Medusa. The name, Algol, comes from the Arabic word al-ghul meaning the ghoul.

Perseus immediately fell in love with Andromeda and asked for her hand in marriage if he managed to save her life. So when Cetus appeared Perseus held the head of Medusa aloft and so confronted the terrible sea monster. The moment Cetus set eyes on the terrible spectacle it was turned into a thousand pieces.

If we follow our imaginary line from Cass this way we come to a bright triangle shape in the West. This is the ‘Summer Triangle’ and signalling that winter is on its way it is starting to disappear in the West. And during the next couple of months it will dip below the western horizon only to reappear again in the East in spring.

The Summer Triangle is made up of stars from three different constellations; Deneb in Cygnus the swan, Vega in Lyra the lyre and Altair in Aquila the eagle.

Starting from Deneb we can trace the shape of the swan; here’s the tail, the wings and here is the long neck leading to the head. This star is called Albireo and marks the head of the swan. To the naked eye it looks like a single star, but if you were to look through a small telescope you would see one of the most beautiful double stars in the night sky consisting of a golden primary star and a blue companion. Although they revolve around each other they are so far apart they actually take 75 000 years to complete a single orbit.

Next we come to the third brightest star in the Northern sky; Vega. Due to the slowly varying movement of the earth’s axis – known as the Precession of the Equinoxes – Vega was once the Pole star, some 10,000 years ago and will be again in a further 12 000 years.

Vega rotates so fast that it is 23% wider at its equator than at its poles and is spinning at 91% of its break up speed. Any faster and it would simply fly apart.

If we take a closer look at Aquila at the bottom of the triangle you might be able to make out a feint band of stars extending through Cygnus, Cassiopeia, Perseus and on to the horizon. This is our home galaxy, the Milky Way. The proportions of our galaxy are staggering; the main disc is 90,000 light years in diameter and 300,000 light years in circumference and the central bulge is 16,000 light years thick.

If you consider that a light year is some six million, million miles you start to get some idea of how enormous the Milky Way really is.

Occasionally you may see a streak of light flash through the sky. This is a meteor or shooting star. Meteors are usually no bigger than a grain of sand and while they are travelling through space at great speeds the can come into contact with the Earth’s atmosphere. This causes friction which in turn heats up the tiny rock until it starts to burn.

Periodically we have meteor showers with meteors apparently emanating from the same point in the sky. These showers typically last from a few hours to a few days and are caused by debris being expelled from a comet’s tail. Each day more than four billion meteors, often miniscule in size, fall from space.

Occasionally one of these objects will survive and hit he ground, these are called meteorites. You can see some meteorites on the space gallery after the show.

The Earth turns and the night slowly gives way to the morning and the only star we can see is our Sun.

Why not try familiarising yourself with the constellations by taking one of our Nightwatch sheets which will not only show you the constellations but also which planets you can expect to see in the Autumn night sky.