Looking at things from a different angle

THERE is often confusion about which astronomical events will be visible from a particular part of the world.

Unfortunately, the answer is not all that simple. In particular, some things are latitude-related, and others are longitude-related.

One feature of the night sky that depends little on one’s position on Earth is how many stars we can see with the unaided eye.

Under excellent conditions, about 6000 stars are seen with our eyes alone, but because we see only half of the sky at any one time, the limit is about 3000.

This does depend, of course, on how much light is spread around the sky.

There are two main factors here: artificial light pollution, and the presence of the moon, especially when it is full or nearly full.

Hopefully you were out watching the eclipse of the moon on Tuesday morning, but you would have noticed that the starry sky was far from spectacular. It was only a partial eclipse, but with only 25 per cent of the moon’s diameter immersed

GEORGE

Australians in the Earth’s shadow, most of the light of the full moon was still present, and the Milky Way would have been virtually impossible to see.

Go out tonight, however, once the sky darkens, and away from the city lights you can obtain a magnificent view of the Milky Way.

The best example of visibility depending on longitude is an eclipse of the moon. Anyone on Earth who can see the moon can see a lunar eclipse, because in such an event it is the Earth’s shadow falling on the moon. Therefore, the moon simply needs to be above one’s horizon.

Because a lunar eclipse typically lasts a few hours, somewhat more than half the Earth can see part or all of a lunar eclipse. On Tuesday morning, this included all of Australia.

However, a solar eclipse is not the same. Every solar eclipse is different, because it is a matter of where the shadow of the moon falls on the Earth’s surface.

In addition, a total eclipse of the sun is visible from only a relatively narrow path across the Earth’s surface.

On August 21 there will be a total solar eclipse visible from a path across the continental United States, but we shall see nothing of that eclipse from Australia because we are nowhere near the path of the moon’s shadow.

Latitude can make a difference in a big way: from a given latitude there are some directions in the sky that we never see.

From Tasmania, we never see the famous star pattern known as the “Big Dipper’’ in the US (or “The Plough’’ in the United Kingdom), because it never rises above our horizon. Similarly, folks in the UK never see the Southern Cross.

This weekend you may learn of a meteor shower called the Perseids, which is one of the most famous of all. It occurs yearly as our planet passes through the orbit of a comet called Swift-Tuttle.

The meteors seem to come from a particular direction in the constellation of Perseus, but that direction is never above our Tasmanian horizon, so we see nothing of that meteor shower here.

In fact, the northern hemisphere is far more favoured for viewing meteor showers. Due to pure chance, the directions from which the meteors seem to come (called the radiants) are, for the best showers, in the northern part of the sky.

So for showers such as the Leonids in November and Geminids in December, we see some of the meteors, but our northern hemisphere friends get a far better view.

Visibility of the aurora is more complex, but in Australia, the farther south, the better, which is why Tasmania is highly favoured for the aurora australis.

Returning to the Milky Way, here in the southern hemisphere we get a far better view of our galaxy than do people up north. This is because the brightest part of the Milky Way is in the southern part of the sky.

So do go out tonight and enjoy the view.

Space

Martin George is manager of the Launceston Planetarium (QVMAG).