Little Big Moon
Thursday, 20 June 2013 07:55
Try and catch a glimpse of the full Moon this weekend, it will be huge!
The Moon doesn’t go around the Earth in a perfect circle – its orbit is elliptical and so sometimes it is closer to the Earth and at others it is further away. On Saturday evening the Moon will be at its closest point to the Earth known as perigee and will be at least 12% bigger than normal.
If you manage to spot the Moon as it is rising or when it is close to the horizon then it will appear even bigger again. This is due to something called the ‘big Moon illusion’. Nobody is quite sure why it happens but the Moon always appears much bigger when it is close to the horizon.
There is no scientific reason why this happens and careful measurement has shown that the Moon is exactly the same size whether it is close to the horizon or high overhead in the sky. It is thought that when it is low down near the horizon you have objects such as trees and houses in your field of view that gives you something to compare the Moon against. But optical illusions aside, the Moon really will look huge over the weekend.
Since there is a point when the Moon is closest to the Earth–perigee – then it stands to reason that there must be a point when it is at its most distant point –apogee. This was on May 11th this year and just happened to occur when there was an eclipse of the Sun. During an eclipse the Moon appears almost exactly the same size as the Sun and so blocks out the Suns disc for a few minutes. During this latest eclipse, because the Moon was at apogee at the time, it was slightly too small to cover the whole disc, instead it left a complete circle of sunlight all around it and the people of central Australia witnessed an annular eclipse.
When the Apollo astronauts went to the Moon they left behind a series of reflective mirrors. Astronomers fire a powerful laser beam at the mirror and time how long it takes for the reflection to return. By doing this they can measure the distance to the Moon very accurately indeed and have discovered that the Moon is moving away from us at about 4cm per year. This might not sound like much but added up over thousands of years it means that in the future a total eclipse of the Sun will not occur as the Moon will be too small to cover the whole of the Sun.
A total eclipse of the Sun will occur on November 3rd this year that will be visible more or less along the equator from Bermuda, right across the Atlantic Ocean and into central Africa. Southern Spain will get to see a partial eclipse while the people of Britain will see nothing of it.
While you are waiting for the Moon to rise in the east at around 22.00 this weekend you may like to turn around and face west and if you have a good view of the horizon you will see a very tight grouping of three planets Mercury, Venus and Jupiter. They will be so close together from 24th to 28th May that you will be able to fit all three in the field of view of binoculars (be sure that the Sun has set first before using binoculars). Jupiter is moving to the far side of the Sun after dominating the night sky all through winter. It will return as a morning object later in the year. Venus is now the brightest planet and as it gradually moves higher in the sky over the next few weeks it will set later and later after sunset and become much more prominent.
This conjunction will be a golden opportunity to spot the tiny and elusive planet Mercury. Because Mercury is so close to the Sun it is never really seen in dark skies, it is always hiding in the twilight sky and so is usually very difficult to spot, but by using the much brighter and easier-to- find Venus and Jupiter it will take you straight to it. Mercury always has a pink tint to it compared to Venus’s dazzling white colour and Jupiter’s yellow. The trio will form an equilateral triangle before separating again.
There will be a very small partial eclipse of the Moon very early on Sunday morning but it is so small it really isn’t worth getting out of bed for!