23 September, 2006

Can someone please explain.......

Yesterday (22nd Sept) was the day that the 'new moon' fell. A new moon has 0% visibility. That's zero, nil, nothing, it cannot be seen. Today is the first day in this moon phase that it might be possible to see a slight sliver of the waxing crescent and even then it cannot be seen until after sunset (today).

So, can someone who knows more about this than me please explain how the crescent of the moon was spotted last night? I am very curious.

Thanks,

HMHB

ps is the weekend in Saudi still Thursday and Friday?

4 comments:

redstar said...

It's amazing how these things always seem to fit in so nicely with the government sector's weekend.

DG said...

My friend, of course the new moon will have zero visibility, especially with so many lights in the city. Try going to a rural area or better still in the mountains & you will be surprised to see the number of stars in the sky, which we city dwellers hardly notice.

Grumpy Goat said...

I'm running the risk of 'dumbing down' celestial navigation here, but here goes anyway.

The new moon occurs when, from the point of view of the Earth, the moon is between the Earth and the Sun. If it's in a direct line there's a solar eclipse. At the exact moment if new moon, the lunar disc is totally dark.

This occurred at 14h45 UAE time on Friday 22nd September. The new moon occurs at the same time everywhere.

As the Earth rotates, it causes the Sun and Moon to apparently track across the sky from east to west. Sunset in Dubai was at around 18h15 on Friday. And moonset was at 18h12. The Sun and Moon are more or less in line, so at a New Moon sunset and moonset must occur at the same time

Now, because of the Moon's monthly orbit, every day at sunset the moon is slightly higher in the sky. Around 13 degrees higher, in fact. So as the Sun and Moon go out of line, the crescent starts to show. If the New Moon is only a few hours old the cresent will be very tiny. Sunlight is so much brighter than the Moon, so unless the crescent is fairly large and/or the sun has set, it is very difficult to see the moon at all.

For the purposes of declaring the start of a new lunar month, it must be possible to see the crescent just before the Moon drops below the western horizon.

I have already noted that last Friday moonset in the UAE was 3.5 hours after new moon. An observer further west, say in Riyadh or Cairo, would get sunset (and moonset) a couple of hours later, meaning that the lunar crescent has chance to get that little bit bigger and therefore more observable.

A side note: A week after the New Moon, the Half Moon should be more or less overhead at sunset. A week after that, the Full Moon is rising in the east at sunset. At the fourth quarter, a further week later, the half moon doesn't rise until about midnight, and doesn't set until midday.

BD said...

^^^ Wonderful explanation. Now, if I could only understand it. For me the crescents come and go when someone tells me so!

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