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

The moon was really big this morning as it hung low on the northwestern horizon - it's almost full. Tomorrow, which is the winter solstice, it will be full and there will be an eclipse as the sun, earth and moon are exactly lined up, so the moon enters the shadow cast by the earth.

This will start at 6:33am GMT (UTC) which is the time in Great Britain and Ireland and will reach its darkest 1 hour 44 minutes later at 8:17. The sun should rise at about 8:35, so it's unlikely we'll see much of the eclipse after that. The whole thing will be finished at 10:01.

For Europe add one hour to these times. For East Coast US, subtract 5 hours - you'll have to get up in the middle of the night to see it. For West Coast US, subtract 8 hours, so that the eclipse starts at 10:33pm on the 20th. Not too late!

The 'totality' of the eclipse, in which the moon is completely in the earth's shadow should last about 70 minutes, but we don't know in advance how dark the moon will be. It depends very much on weather conditions on Earth. The moon make turn black, grey or red depending on how much light gets refracted through the earth's atmosphere.

Eclipses are quite common - there are about 3 every year. Total ones where the moon goes completely dark are rarer, and a total eclipse happening on the winter solstice is very rare - the next one will be in 372 years.

When the moon is full is it full everywhere on Earth?

I have sometimes wondered if there are places where the moon isn't visible at all. I mean, if it described quite an arc directly over me, would it not be missing from the southern hemisphere?

Meanwhile, we continue to have very slow and lazy flurries, so I don't expect a clear sky at 6'ish in the morning ...

I don't think so. I've heard that Muslims celebrate festivals by the local view of the moon's phase, and this can happen on different days in different parts of the globe.

When the moon is full, it is full everywhere in the world. The moon is far enough away that it looks the same wherever you are in the world. It might be day in one part of the world and night in another, but if you can see the moon at all, then it will look the same.

Since the moon is on only one side of the earth at a time, it is only visible from 50% of the world at any moment. The moon rises and sets similarly to the sun, but every 24 hours and 50 minutes rather than every 24 hours.

The Moon is 250,000 miles away from us, give or take, and our Earth is only 8,000 miles across, so you are talking about an angular distance of less than 2 degrees. For all intents and purposes it would be full at the same time for everyone.

Maybe a new moon is discernable, though?

I think the Muslim thing is just that since the new month starts when the new moon is sighted, for consistency across the Muslim world, they insist on it being the astronomers in Mecca who officially see the new moon first. That way we don't have the month changing on different days in different countries.

As it is, the new month usually starts when predicted, but very occasionally they don't see the new moon at the predicted time and the old month has to go on for another day, causing great confusion, particularly since the calendars are all printed in advance with the Muslim calendar date and Gregorian calendar date on them. These are all then wrong for the rest of the month.

*takes notes and sets alarm clock*

Thanks Gnomon - that clears it up

A very rough, back of the envelope calculation seems to suggest that the whole Earth moves through the exact Full Moon phase in just over 4 minutes. I'm open to correction on this.

Discerning a new Moon would be hampered by where the country is and whether the Moon is even visible from that location when the event occurs. Another factor is the difficulty of seeing the Moon at all when it is so close to the Sun's position in the sky.

Actually, strike that: it takes about 7 minutes.

I think.

We haven't had a clear sky for ages, so we won't notice any full moon or eclipse.

But if we did have a clear sky here, Gnomon, would be be able to see the eclipse?

Oh - just a minute, I just have to check what time the moon rises and sets tonight, don't I?



Ah! For anyone else in this area, I've just found this useful timetable. All we need now is the clear sky:

•Moon rises 20.12.2010 at 15.27 h Central European time
•Sun sets on 20.12.2010 at 16.19 h MEZ
•Pre-eclipse darkening begings at 06.16 h CET on 21.12.
•Lunar eclipse first visible from 07.00 h CET
...up to
•Maximum complete eclipse at 09.17h CET
•End of main phase at 09.53h CET
•End of visible eclipse 11.35h CET

For those in North America, here's Phil Plait's guide to the eclipse: http://is.gd/j1AaA

good point, TC. If the moon is directly opposite the sun, then when the sun is up, the moon is down. So it's not just that the moon will be hard to see once the sun rises. It is that the moon will be below the horizon. So you should be able to see the eclipse, but the moon will be very low above the horizon, and then the sun will rise and the moon set, ending the show.

However, it is beautiful to see the moon in eclipse at the horizon. It makes it look so much larger.

Oh, and if anyone's interested, our maps of the lunar surface are getting MUCH better: http://www.nasa.gov/mission_pages/LRO/news/lola-topo-map.html

Let's see now.

At 6:30 tomorrow morning, the temperature is forecast to be -9 degrees C. I think I'll have skip this lunacy.

B

But think of the excitement, Mu Beta! You'll look up and won't see the moon, because it is being eclipsed.