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View Full Version : Stuck on an astronomy question if anyone can help...

Chica
2012-Oct-24, 04:37 PM
I'm doing an advanced astronomy course in the hope of making it back into uni as a mature student but a few of the questions are completely baffling me (as I have no prior experience of astronomy).

The one I'm really stuck on reads 'Given that the moon' synodic period is 29.53 days, find the phase of the moon four and three quarter days after new moon.'

I don't need someone to answer the question, I just need a nudge in the right direction if possible :o

antoniseb
2012-Oct-24, 04:45 PM
I'm not sure what you're missing, and assume you know most of this... so here's a start: If you measure precisely the phases are not exactly 29.53 days apart due to the non-circular sun-disturbed nature of the lunar orbit, but the question you have seems to be assuming averages and treating it like a circular orbit.

So the basic question is if the Moon's phase makes a complete circle (360 degrees) in 29.53 days, how many degrees does it complete in 4.75 days? Your teacher or textbook may have some other way than degrees for expressing phase. Phase angle usually starts at 0 for the new Moon.

Chica
2012-Oct-24, 05:19 PM
But in the synodic period of 29.53 days doesn't the moon move more than 360 degrees?

antoniseb
2012-Oct-24, 05:59 PM
But in the synodic period of 29.53 days doesn't the moon move more than 360 degrees?
Sure, the Moon moves more than 360 degrees, but we are talking about phase angle with respect to the (moving) Earth-Sun line. Actual absolute rotation with respect to the distant galaxies doesn't matter for this calculation. So, the Synodic period is all that counts for this calculation.

Chica
2012-Oct-25, 06:45 AM
Okay, so it's 360/29.53 and then multiply by 4.75 which gives 57.9, but we are given the actual answer and it's 0.2343. Very confused :(

chornedsnorkack
2012-Oct-25, 08:56 AM
Okay, so it's 360/29.53 and then multiply by 4.75 which gives 57.9, but we are given the actual answer and it's 0.2343. Very confused :(

But that seems to give a clue to the definition of phase.

When the phase of the Moon is 180 degrees (full moon), 1,000 of the disc is shining. Also, cos 180 is -1,000.

When the phase is 0 degrees (dark moon), 0,000 of the disc is shining. Also, cos 0 is +1,000.

When the phase is 90 degrees (half moon), 0,500 of the disc is shining. Also, cos 90 is 0,000.

So, which part of the lunar disc is visible when the phase is 60 degrees?

Cos 60 is +0,500.

Therefore, 0,250 of the disc is shining.

Is that the right logic?