Eyes on the clouds

I HAVE, on many occasions, been asked by older visitors to the Launceston Planetarium why they do not get as good a view of the starry night sky as they remember when they were children.

A major reason is the increase in the light pollution from our cities: the population centres are gradually becoming brighter and brighter, and this makes it hard to see the less bright objects in the sky — especially the fuzzy glows of light such as the Milky Way.

A solution to that particular problem, of course, is to take a country drive on a clear, moonless night. Getting away from so many artificial lights makes a huge difference to our view of the sky.

However, there are other things to think about. At this time of the year, sunset is quite late, and the end of twilight much later. We have the additional problem of daylight saving, which adds a full hour to the time at which we can obtain our best view of the night sky.

Tonight, for example, the end of astronomical twilight, which is the moment that astronomers consider the very last glow of twilight is finally gone, occurs at 10.42pm for Hobart, 10.33pm for Launceston and 10.35pm for Burnie.

This moment occurs when the sun’s altitude is 18 degrees below the horizon. This means that if you drew a line from the horizon to your eye, then back down through the Earth in the direction of the centre of the sun, the angle at your eye would be 18 degrees.

Waiting until quite late in the evening and getting away from the city lights are well worth doing, but there is another very important consideration: the way our eyes work.

I wrote an article on this a few years ago, but it is timely to remember some important facts — and I shall get you to test them out this time, by giving you a couple of great objects to see.

As you have no doubt noticed, your eyes take time to get used to the darkness. When placed in dark conditions, the iris in our eye increases the size of the pupil, letting in more light. This takes place fairly quickly, but over a longer period, the retinas of our eyes gradually become more adapted to the dim conditions.

In particular, the lightsensitive cells in our eyes called the rods start to become important after several minutes, boosting our ability to see in the darkness. However, the rods are located around the edge of our vision, so this means we can see dim objects away from the centre of our vision more clearly than if we look directly at them.

Now let’s try this out. In the evenings at this time of the year, the Milky Way is far too low in the sky to obtain a good view of it, but the opposite is true for two patches of light in the southern sky called the Magellanic Clouds.

They were named after the explorer Ferdinand Magellan, but of course they were well known to people of the southern hemisphere long before the voyages of European explorers.

They are, however, not clouds — this is just a term that we use for them. They are two of the nearest galaxies to our Milky Way Galaxy, in orbit around us.

So go outside, preferably away from the city lights, face to the south, and wait for your eyes to become used to the darkness (preferably for at least 10 to 15 minutes). If you look high in our southern sky, you will see these objects as two patches of light, like little faint clouds several times larger than the apparent size of the full moon.

Once you have located them, look a little away from them and you will see how the periphery of your vision is better at revealing faint objects at night.

The technique is called averted vision by astronomers, and it is a powerful tool to help you see more in the night sky.

We received exciting news this week that NASA’s latest Mars mission, called Insight, landed safely on Mars at 7am Tuesday our time. I shall have more to say about that soon!