Astronomers do it at arm’s length!

Torbay Weekly

To get the best out of the time spent viewing the night sky it is useful to learn how to find your way around.

This is true for telescope users as well as naked eye observers.

I once had a message from two well-equipped but novice astronomers telling me that they had spent a couple of hours setting up their telescope and tracking down the planet Uranus.

I congratulated them and joined them the next evening when they wanted to also find the planet Neptune.

Once again, they started to set up their equipment. I set up my telescope and within 20 minutes had found both, Uranus and Neptune. They were amazed - I never told them that I had studied the star chart before arriving.

Astronomers first try to learn the patterns of the major constellations as a guide.

The Plough and the W of Cassiopeia are good all-year signposts.

At this time of year we also use the Sickle or back-to-front question mark of Leo.

In the summer, the long cross shape of Cygnus is conspicuous. And in the winter the three stars that form the Belt of Orion, along with his hourglass shaped outline, make excellent signposts.

This article will return to these in the appropriate seasons.

The next trick astronomers use is 'star hopping'.

Starting from a known bright star simply hop to the next stars in a pattern to lead you towards the general area of a fainter object. Objects such as the Andromeda galaxy can be found with binoculars using 'star hopping'.

We will return to this in the most appropriate months.

The last trick is the rule of 'arm’s length'. This is based on the approximation that everyone’s hand is the same size at their own arm’s length.

Using this, astronomers have devised a 'rule of thumb' method of measuring angular distance across the sky.

Angular distance is often used in magazines and technical sources to give the positions of objects relative to each other.

So, at arm’s length the width of your little finger is approximately 1⁰. The Full Moon (and the Sun) only covers 1/2⁰ of sky so your little finger should completely cover the Full Moon with ease.

The thickness of your thumb (at arm’s length) is 2⁰. The distance across your three middle fingers is 5⁰ and the distance across a clenched fist (knuckles up) is 10⁰.

Then, still at arm’s length, the span between your forefinger and little finger is approximately 15⁰ and your widest span from thumb to little finger is approximately 20⁰.

The distance from the zenith (overhead point) to the horizon, in any direction is 90⁰.

Using this information and these tricks, you will be able to begin to find your way around the night sky for yourself.

Of course, nowadays you could just download an app on to your smartphone, some of which are free, which will identify constellations and objects for you but there is no skill or sense of achievement in that.

The Star Chart

The sky will look like the chart on May 5 at 9pm and again on May 21 at 8pm. And four minutes earlier on each successive night e.g. 8.56 on May 6.

To use the chart hold it above your head while facing south so that you can look directly from the chart to the sky.


Please note all times given in this article are in GMT so remember to add an hour to get the time in BST.

Mercury and Venus: Both of these inner planets are closer to the Sun than the Earth so are never very far from the Sun (especially Mercury).

Mercury and Venus are both visible this month. On May 4, Mercury will lie close to the Pleiades cluster in the background constellation of Taurus, in the western sky, half-an-hour after sunset and by May 28 will be joined by Venus (much brighter) in the north-western sky also half-an-hour after sunset.

Because they are never far from the Sun it is best to wait until the Sun dips below your horizon then scan the sky from that point westwards. NEVER LOOK DIRECTLY AT THE SUN with an optical instrument!

Mercury is quite difficult to find but should be much easier with Venus and the Pleiades as markers.

Mars: The red planet is still visible but is getting fainter as it moves away from the Earth. It may be seen against the background constellation of Gemini, this month.

On Mars, the Ingenuity drone has - at the time of writing - taken two test flights.

These are not only to test the flight capability in Mars’ thin atmosphere, but also to test the programming capability of the operators back on Earth.

The instructions for each flight are written on Earth as a chain of commands and sent to the drone on Mars.

Continual commands are not practical as it takes nearly 13 minutes for each signal to reach Mars.

Jupiter: At the end of the month Jupiter lies just about 15 degrees above the south-eastern horizon, half-an-hour before sunrise.

Saturn: The ringed planet can also be found about 15 degrees above the southern horizon half-an-hour before sunrise.

The giant planets will be better placed against dark evening skies later in the year.

Uranus and Neptune: These planets are not visible this month.

Meteor shower: The Eta Aquarid meteor shower - so called because the point they appear to radiate from lies near the star Eta Aquarii - peaks on May 4 but this is a faint and unreliable shower.

It is still possible to see sporadic (non-shower) meteors like the Cotswold meteor of last month.

Comet: There is a comet in our skies during April, but it is very faint and will need a telescope to be seen.

This is comet 7P Pons/Winnecke, named after the 19C French astronomers who discovered it.

It is a periodic comet which means it returns every so often - approximately 93 years, in this case.

It passed so close in 1927 that it was feared that it might collide with the Earth.

'Close' is an astronomical term – the comet came within 40 million miles of the Earth while the Moon is only ¼ million miles from the Earth.

So, the comet was about 1,600 times further away than the Moon however, relative to astronomical distances, this is regarded as 'close'.

The Last Quarter Moon occurs on May 3 with New Moon on May 11. First Quarter follows on May 20 with Full Moon at the end of the month on May 27.

Data supplied by Simon Harding, observations secretary for Torbay Astronomical Society

Diary Dates

The next meeting of the Torbay Astronomical Society, complying with the current Covid regulations will be online via Zoom.

For details contact the secretary TAS on . Visitors and prospective members especially welcome.