Moon half illuminated by Sun - but not as expected ! Astronomy question.?

Question

Next time you seen the Moon at sunset on a clear night, have a closer look. You might expect the bright half of the Moon to be disected by the line between the centres of the Sun and Moon. But it's not !
Any Astronomy experts out there ?

Thanks all you guys. Slartybartfarst has what seems the most likely answer. Even so the effect seems larger than I might have expected from refraction. But then if you designed the Fjords you probably won't be wrong on this. Thanks.

Answer 1

Atmospheric refraction makes positioning objects inaccurate near the horizon. Especially with a thermal difference making air have different refractive indices. it is like seeing a ship that is actually over the horizon it gets refracted nearer and in sight! I is called a mirage amongst other things.

If you now disect the moon's illuminated side and extend you will get a slightly more accurate position for the sun however remember that the moon itself might have a mirage position!

You others note:
Theres nothing wrong with the wording of the question. The questioner has used classic geometric description.


Thank you. The effect isn't large however consider a slight error in how you position the sun and the fact that the light from the sun has travelled such a long way. If you extend the line disecting the lit side of the moon right out as far as the sun you might by then be as far of as almost an astronomical unit! Or something of that order.

By the way an excellent observation!

Answer 2

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Answer 3

Are you sure it isn't? Did you measure it? It can appear very deceiving if you just try to judge it by eye. A lot of astronomical phenomena are like that. For example I have been fooled into thinking a star or planet was nearly at the zenith, but when checking it's true position with star charting software I found it was only about 60 degrees in altitude!

Answer 4

i'd hardly say i'm a astronomy expert, i have an astro GCSE if that helps.

i don't really get your question tbh. if ya mean that you'd expect the unlit and lit sides of the moon to be the other way round, its probably something to do with the unusual positioning of the moon at the mo or the way the sun and moon are positioned around earth.

if ya cud possibly reword the question later on adn submit that one on here, i cud have another crack at it. sorry, i guess this doesn't really help.

interesting tho, i'll look at it soon - never really questioned it before now....

Answer 5

When the sun is at sunset its real position is not the one you see, because of the effect of the atmosphere. Therefore you can't judge correctly its alignment with respect to the moon which is on the opposite side of your field of vision.

Answer 6

It's disected by a line (roughly) PERPENDICULAR to the line sun-moon.

But of course, the moon being (roughly) spherical, that line looks curved from our point of view. But it's quite normal: the moon is spherical.

Answer 7

could you reword that? From what i got, the sun is larger that the moon thats why this happens

Answer 8

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