Without a shadow of a doubt

EARLY on Tuesday morning, well before the sun rises, people all over Australia will be able to see a partial eclipse of the moon, weather permitting.

Eclipses of the moon take place when the moon passes into the Earth’s shadow in space, and they can happen only when the moon is full.

It is easy to see why. The Earth’s shadow always points directly away from sun, and it is when the moon is in this location that it is opposite the sun in the sky and fully illuminated.

You may wonder why we don’t see an eclipse of the moon every month. This is because the moon’s orbit is tilted by about five degrees compared with the plane of the Earth’s orbit in space.

Most of the time, therefore, the moon passes “above’’ or “below’’ the Earth’s shadow. Sometimes the moon is immersed completely within the shadow, resulting in a total lunar eclipse. Such an event was most recently visible from Tasmania on April 4, 2015, although many parts of the state were cloudy.

At other times, however, the moon skims along the edge of the shadow, resulting only in a partial eclipse.

It is quite safe to watch an eclipse of the moon, which occurs at night. This is not so with solar eclipses, seen during the day, because it is dangerous to look directly at any part of the disc of the sun at any time.

It is great fun to watch lunar eclipses with binoculars and telescopes. My choice is a good pair of binoculars, which gives a great view of such events.

Lunar eclipses offer a great chance to see the curvature of the Earth.

Because the Earth is round, its shadow is also round, and this curved edge is also rather more apparent in binoculars.

Whenever a lunar eclipse is coming up, showing beyond a shadow of a doubt (pardon the pun) that the Earth is round, I often think about the ideas, held mostly long ago, that the Earth was really flat.

It’s certainly not obvious, by simply making observations from the ground, that we are standing on a spherical planet. Indeed, there are still some adherents to this idea — although they are generally not serious.

During Tuesday morning’s eclipse, only at most 25 per cent of the moon’s diameter will be within the Earth’s shadow.

The eclipse starts at 3.22am, and reaches its maximum at 4.21am. After that, the section of the moon that is darkened will gradually emerge from the shadow, and by 5.19am the moon will once again be fully illuminated by the sun.

You may notice, just before and after the eclipse, that part of the moon appears to have a very slight shading. This is caused by the lighter part of the Earth’s shadow, called the penumbra. However, the penumbra still allows direct sunlight to fall on to the Moon. It is only during the main, umbral part, that part of the moon is completely cut off from the light of the sun.

In just over two weeks, I am sure that you will see plenty in the media about an eclipse visible from the US. That one will be a total solar eclipse, in which the moon completely blocks out the sun during the daytime along a relatively narrow path across the Earth.

It’s no coincidence that this will be occurring so close in time to our partial eclipse of the moon. The orbital mechanics often result in a lunar eclipse being followed a fortnight later by a solar eclipse, and vice versa.

The next total solar eclipse to be seen from Tasmania will not occur until the year 2131, but total lunar eclipses are more common from a given place.

Our next total lunar eclipse will be seen on January 31 next year, less than six months away.

Let’s hope for clear skies on Tuesday morning.