Stargazing April: getting to know our mysterious neighbour the Moon

Check out the sensational lunar craters; also, it's a good time to catch Leo, the celestial lion

The lunar far side is far more cratered than the near side (AFP/Getty)

O Moon, when I gaze on your beautiful face
Careering along through the boundaries of space
The thought has often come into my mind
If ever I shall see thy glorious behind.

So wrote the poet Sir Edmund Gosse in the 19th century – although he attributed the words to his housekeeper.

Gosse’s query is one of the Moon mysteries we’ll be looking at in this month’s column. For a object so close to us – it orbits just 384,400km away from the Earth – our knowledge of our “natural satellite” is still incomplete.

Let’s tackle the Moon’s “behind” first. One of the biggest misconceptions in astronomy is that the far side of the Moon is the “dark side”. It isn’t. As the Moon orbits the Earth, all of its surface catches the Sun’s light in turn. We see only one face of our satellite – the near side – because the Earth’s gravity has “braked” the Moon’s spin.

The lunar far side is far more cratered than the near side (more about craters later). The first humans to get up close and personal to the Moon’s “behind” were the crew of Apollo 8, in 1968. Lunar module pilot Bill Anders came up with a great description: “The back side looks like a sand pile my kids have played in ... it’s all beat up, just a lot of bumps and holes.”

The later astronauts who landed on the Moon have helped to unravel a major puzzle: where did the Moon come from? Compared with other moons in the solar system, it’s surprisingly large relative to its planet, fully one-quarter the width of the Earth. But moon rocks brought back by the Apollo astronauts may have answered the question. Although not an exact match, their composition is very similar to rocks in the Earth’s surface.

This leads to one conclusion: that the Moon was literally blasted out of the Earth.

The young solar system was a place of incomparable violence. Worlds collided. And 4.5 billion years ago, a wayward world headed our way – hell-bent on destroying the young Earth. This Mars-sized body hit its target. But Earth held its ground, pulling itself back together as a globe of molten lava.

And the impact had a permanent legacy. Molten rock splashed into space, forming a fiery ring around the Earth. These droplets came together to create the Moon. This “big splash” theory explains why the Moon is so large: it’s the end product of two worlds.

The bombardment from space continued after the Moon was born. The result was a plethora of impact craters, some hundreds of kilometres wide. Earth, too, has craters – Meteor Crater in Arizona is the best preserved – but most have been eroded by weather and geological activity. The Moon has neither, so its craters are perfectly preserved fossils of the past.

The night sky at around 11pm this month

The best time to view the craters is when the Moon is half-lit; the sunlight comes in side-on, and casts long shadows. The lunar craters look sensational, even through the smallest of telescopes.

You need only your eyes to see the “face” of the Man in the Moon (or the Hare in the Moon, according to some cultures). When the Moon is full, you’ll find its disc splattered with enormous dark basins. Early astronomers – peering at them through the first telescopes – naturally thought they were vast stretches of water, and the Latin name mare (“sea”) has stuck ever since.

The maria (plural of mare) are the culmination of the Moon’s battering. Some 3.8 billion years ago, huge asteroids rained onto our satellite’s surface in the “Late Heavy Bombardment”, blasting out these huge scars. Most are bigger than 700km across; the largest – Oceanus Procellarum – is 2,568km in diameter. The impacts penetrated deep inside the Moon itself. Magma welled up inside the basins, filling them with lava that cooled to create a smooth, dark surface.

Nasa recognised that the maria were ideal sites for the Apollo landings. But will astronauts ever walk on the plains of the Moon again? Almost certainly, yes. Future moonbases (or even moon villages, based on colonies in the Antarctic) will allow scientists to explore deep space under airless skies, as well as practising for living on Mars in the future. Ice – which lies in deep shaded craters at the Moon’s poles – could supply fuel for space vehicles, as well as oxygen for breathing.

And the ultimate? Space hotels for lunar tourists! A number of hotel chains have talked about it, but there’s been very little action yet. Anyone for a super-cooled G&T?

What’s up?

There’s a giant feline on the prowl these April evenings. Look high up in the south to see the distinct shape of the celestial lion, Leo. The bright star Regulus lies at his heart; a curve of stars above (like a back-to-front question mark) depicts his chest, neck and head; while a triangle to the left marks out his hindquarters, terminating in the star Denebola – “the lion’s tail” in Arabic.

Ancient Babylonians saw the pattern to the lower left of Leo as a furrow in a field, and called the bright star here Spica, meaning “ear of corn”. Later, the Greeks and Romans placed their pure goddess of justice in this part of the sky, and named the distinctive Y-shape of stars Virgo, the virgin.

To the upper left of Leo and Virgo you’ll find Arcturus, the fourth brightest star in the sky. Scrutinise it carefully (binoculars help), and you can see that this cool red giant star shines a distinct orange colour, while hotter Spica and Regulus are blue-white.

In the evening sky, we also have two planets that outshine any of the stars. After sunset, you can’t miss brilliant Venus, hanging in the western sky. It puts on a couple of showstoppers this month: first with the crescent Moon on 17 April, and then passing the Seven Sisters (Pleiades star cluster) on 24 April

In the opposite direction, the second brightest planet, Jupiter, rises below Arcturus around 10pm. Wait around till the early hours, and you can catch another pair of planets rising in the southeast: reddish Mars and slightly duller Saturn.

You can watch the Moon pass in front of the Hyades star cluster (near the bright star Aldebaran in Taurus) on 18 April. And we’re expecting celestial fireworks on the night of 22/23 April, when fragments from Comet Thatcher burn up in the atmosphere as the Lyrid meteor shower: best to watch in the early hours after the Moon has set.

Diary:

2 April, morning: Mars near Saturn

3 April: Moon near Jupiter

8 April, 8.18am: Moon at Last Quarter

16 April, 2.27am: New Moon

17 April: Crescent Moon near Venus

18 April: Moon occults Hyades star cluster

22 April, 10.46pm: Moon at First Quarter; Lyrid meteor shower

24 April: Venus near the Pleiades

29 April: Mercury at greatest western elongation

30 April, 1.58am: Full Moon, near Jupiter

For the low-down on all that’s up in the sky this year, check out Heather Couper and Nigel Henbest’s latest book, ‘Philip’s 2018 Stargazing’