h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The International Year of Astronomy begins its second MoonWatch week, tomorrow on the 24 October.

There are few things in nature more beautiful than a crescent moon in a clear sky and MoonWatch week is geared to bringing our nearest planetary neighbour a bit closer and a making it a little more familiar.

Why this week? Well, for observers in the UK and northern hemisphere, the moon’s path across the night sky during the autumn and winter gets further above the horizon as the ecliptic gets higher during the night time hours. In October it is becoming better placed and appears at a reasonable hour in the early evening for viewing some of its most dramatic scenery.

This evening (Friday 23rd) was a good start with a five day old moon about ten to fifteen degrees above a clear horizon. That’s a full waxing crescent showing in the early evening sky. Weather permitting, as the month progresses, it will get higher and more prominent, later enlarging through first quarter and some of the way on to full moon. As it does so, it reveals some of the most dramatic of its features and is well worth a close up look. Up to, and just past first quarter, a telescope reveals the remains of cataclysmic events that shaped a planet’s surface. For the most part these events occurred some 3.5 to 4 billion years ago and have remained largely undisturbed ever since.

The Moon’s features are best viewed along the terminator. That’s the line that divides the illuminated side from that part that’s still in shadow. As the week progresses the terminator moves across the surface and the low angle that sunlight is reflected from the surface near the terminator throws the features into a stark and dramatic relief.

It’s time to get the telescope out.

Deke

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Last night was too cloudy (again, I've missed the Orionids maxim now) and today it's raining, forecast: rain, rain, rain

Sometimes when it’s cloudy/wet/cold like this, the weather occasionally clears up a bit as the Sun goes down and it gets cooler. It was beautifully clear yesterday evening just after sunset, right down to the horizon. I won’t hold my breath, but I will poke my head out of the door this evening, just in case.

I spent about a half hour outside each evening Orionid hunting and an hour after midnight on the night of the peak. Didn’t see a thing. Next morning at work a colleague told how he’d been on the way to work, before dawn and seen a ‘really bright flash of light in the sky that went right over his head’ Did I know what it was? From his description it might have been one. That’s one more than I’ve seen.

Deke

Have you noticed how these clueless people get to see the best things?

I actually spotted Jupiter about an hour ago, and I'm going to take a peek outside before I retire for the night,

MoonWatch Week, Day 1

I spent a happy half hour perambulating up and down the terminator this evening.

One of the more interesting features is the trio of craters uncovered yesterday, Theophilus, Cyrillus and Catherina. Part of their floors are still in shadow and the central peaks of Theophilus show up well with a long shadow. They border Mare Nectaris, and just to the south is Rupus Altai a chain of mountains that is all that is still easily identifiable of the pressure ring that formed as a result of the impact that formed Nectarus. 500 Km long and with a scarp face that drops some 2,000 meters on the inner side.

On the southern end of the terminator passes through the southern highlands. Highly cratered and pretty much all that is left of the original face of the moon on the visible side.

Deke

I watched the crescent moon slide past Jupiter tonight, at varying times.

Too much cloud for me and it keeps raining - that really annoying and soaking drizzle.

t.

MoonWatch Day 3

The terminator has moved up to uncover some of the giant craters, although they are more properly described as 'Walled Plains'. They are big enough to be identified as craters, but distinct from the great plains of lava flooding in the basins which we identify as 'Seas'. If you were standing in the middle of one of these and looking at the horizon, you wouldn't be able to see the mountain ranges that surround you. That's because the curvature of the lunar globe is greater on the smaller lunar body and slopes away until the distant mountains are below your horizon.

Two of the granddaddies of the walled craters are easily picked out, even with binoculars. Hipparchus and Albategenius. Albategenius is the younger and the more prominent. Hipparchus is named after the Greek astronomer, who created the first star catalogue, immortalised in Ptolemy's Almagest. It lies just to the north-east of Sinus Medii, the centre of the observable face. The crater is old and has been flooded by lava flows. Most of its wall has been worn down and breached where ejecta flow from the creation of the Imbrium basin flowed through it.

Directly south is the much more clearly defined Albategenius.Again it's a flooded crater and has a central mountain peak towering some 1500 meters above the plain. Another much later crater has implanted itself in the south western wall.

The southern highlands south of these two contain a plethora of craters many rivalling the above in size and complexity. North of Sinus Medii the Mare Serenitatus is almost completely exposed. The border line is Montes Apenninus and Montes Caucasus which divide Serenitatus from the Imbrium Basin which will come to be exposed in the next day or so..

Deke