Shropshire's Sky at Night - May

Shropshire stargazer Carl Drinkwater tells you what to look out for in the skies above in May – without the need for an expensive telescope:

Venus, Saturn and Mars

These planets are still visible this month, but they are definitely all past their best. All three get fainter as May goes on as not only do they get further from Earth, but the Sun is setting later each day.

You can see Saturn close to Spica, between south and south east at 10pm all month. They appear to move southwards as the month goes on.

Mars is a lot higher in the sky, diagonally up and to the right from Saturn, and can be found between south and south west at 10pm.

Finally, Venus is still very bright, but is low in the western horizon as the Sun sets. You should be able to see it from about 8:30pm at the start of the month, but gets noticeably closer to the Sun as the month goes on and will soon be lost in the twilight.

Super Moon

The term "Super Moon" isn't really an astronomical definition, but is widely used by news outlets. It's generally considered to be when the Moon's perigee coincides closely with a full moon.

The orbits of the planets and moons aren't circular nor concentric, and so the distance between the Earth and Moon varies slightly as the days and months go by. Perigee is the point where the two are at their closest, and apogee is the point where they two are at their furthest. The Moon orbits the Earth once a month, so there is one perigee and one apogee every month.

6th May 2012 marks the perigee of the Moon, and coincides with the full moon in May to within one hour.

Because the Moon is closer to the Earth than on average, and it is a full moon, it is bigger and brighter than normal, hence the name "Super Moon". But is it really noticeable?

Here is an image which shows the difference between the full moon on 6th May 2012 (on the left) with the full moon that occurred on 20th December 2010 (on the right.) The bigger full moon is about 8% bigger in terms of radius, and about 16% larger in terms of area, which is pretty significant. (The difference can be up to 10% and 20% respectively.)

However, almost everyone overestimates the size of the Moon. Try this interesting experiment. Pick a night where you can see the Moon in the sky. While still inside, hold your arm out with one eye closed, and try and guess how many fingers you'd need to completely cover the Moon. Your middle three fingers? Two fingers? Just your thumb? Now go outside and see what you need to cover the Moon. I think you'll be surprised. Scroll down for the answer if you don't want to try it yourself.

You can see that it isn't really going to be noticeable in terms of size, but if it's clear, you might notice that it is brighter than normal.

The super moon is separate to an optical illusion known as the "Moon illusion." The Moon illusion is probably why you overestimated the size of the Moon. The Moon looks much larger when it's closer to the horizon as it does when it's higher in the sky.

The Moon rises to the south east at 10pm on 6th May. If you go outside just as it's rising over the horizon, and try the above experiment by trying to cover it, you'll be even more surprised.

The explanation of this illusion is debated by scientists, with several explanations being put forward.

Another Meteor Shower

The Lyrids last month were pretty much clouded out for me, although I did manage to see a few meteors through breaks in the cloud, even managing to capture one of them on camera.

There is another meteor shower on the nights of 5th and 6th May, called the Eta Aquarids. Unfortunately, as mentioned above, there is a particularly bright full moon at the same time, which means few meteors will be visible. If you're outside looking at the Moon though, keep an eye out for them though.

The meteors will appear to come from the constellation of Aquarius, which is actually below the horizon the very early hours. Therefore, any meteors you do see will look like they are rising up from the eastern horizon.

Annular Solar Eclipse

There is another big astronomical event featuring the Moon in May, which further demonstrates that orbits aren't perfectly circular. There is an solar eclipse on the 20th/21st May.

A solar eclipse occurs when the Moon passes in front of the Sun, and causes a shadow to fall on the Earth's surface. The Sun and the Moon appear to us to be about the same size, so when the Moon can cover the Sun perfectly. When it does cover the Sun perfectly, we have a total eclipse.

As we've seen, the size of the Moon does vary, and so does the Earth's distance to the Sun. On those occasions when the Moon does pass perfectly in front of the Sun, but isn't quite large enough to completely cover it, it is an annular eclipse, and it's an annular solar eclipse that is happening this month. From Earth, it will look like the Sun is a ring of light around the Moon.

Unfortunately, it won't be visible from the UK! The best places to see the eclipse are Japan and the West coast of the United States. The next eclipse visible in the UK will be in 2015, but it will be only a partial eclipse. However, over 80% of the Sun will be covered, so should be quite spectacular.

Covering the Moon Spoiler!

So just how many fingers to do you need to hide the Moon? You can cover the Moon using just the nail of your little finger.