The Andromeda Galaxy lies 2.2 million light years away, and a light year is almost 10 million million kilometres. The light we see tonight left Andromeda when the first humans were walking the Earth. M33 is slightly more remote, some 2.4 million light years from us. It is the most distant object you can see with the naked eye.
With the Milky Way - our galaxy - these two are the biggest members of a small swarm of galaxies called the Local Group. Giant of the group is Andromeda. It is almost half as large again as our Milky Way, and contains twice as many stars - about 400 billion. Long-exposure photographs reveal many of these stars ordered into a set of dim spiral arms. The view through a telescope, though, is disappointing. You can see the galaxy's oval centre fairly easily, but the arms are too faint to see with a backyard telescope.
M33 is more of a tease. You may see it with the naked eye on a really clear night, and low-power binoculars should show it as a large, dim circle of light. But turn the power of an amateur telescope on M33 and it disappears altogether. Its light is so diffuse that the slightest magnification spreads the light out to the point of invisibility. But it is even more photogenic than Andromeda. Wrapped around a small central hub, long exposures show loosely wrapped arms of young blue stars, strung with pink nebulae like rubies on a necklace.
If moderation in all things is to be desired, our Milky Way fits the bill perfectly. It is not too big, not too small, not too dull, not too flashy. Goldilocks would undoubtedly find it "just right".
Planets and stars
Early risers can expect one of the year's best views of Mercury - if they get up at 6am mid-month. There will be nearly two hours to inspect the elusive innermost planet before the Sun comes up. Venus is not worth wasting your energy on. It will be putting on a much better show early next year. Mars, too, is immersed in evening twilight.
Jupiter is still worth looking out for, although it is much more distant than it was at "opposition" in June. It sets in the south-west a couple of hours after the Sun. And Saturn - in a barren area of sky - is on view nearly all night long. Its glorious rings are almost "edge-on", and in a small telescope, the planet appears ringless.
One of the more exciting solar system sights this month is the appearance of the Orionid meteor shower. These minuscule fragments of Halley's Comet rain through the atmosphere between 16-27 October, peaking on the night of 22 October. Observers in a dark spot may see up to 25 fast-moving shooting stars an hour.
On 24 October, those in a line of countries from Iran through India to northern Borneo will see the Sun totally eclipsed by the Moon for up to two minutes. In East Africa and Asia, the eclipse will be partial.
Diary (all times BST)
1 Oct 3.36pm Moon at first quarter
8 Oct 4.52pm Full Moon
16 Oct 5.27pm Moon at last quarter
22 Oct Peak of Orionid meteor shower
22 Oct 2am British Summer Time ends
24 Oct 5.37am New Moon; eclipse of Sun centred on IndiaReuse content