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Daytime Moon


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Evening all,

Was gazing up earlier today at around 15:30 and noticed a half moon in the east-ish.

In the west-ish there was of course the sun.

My question is why was only a half moon visible?

It's clearly something obvious I'm missing but to my tiny little mind, if there's nothing in the way between them, the moon should appear full.

So what am I missing? :(

Thanks in advance for putting me in my place :o

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Hi, at that time this afternoon, the moon was quite low in the East and the Sun quite low in the West.

So there was a good chunk of the Earth inbetween the two.

If they were higher up in the sky or both on the same 'side' (E or W) then the moon would have been full(er).

I hope you understand this explaination; I know what I mean.:o

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Hi Starjust

The easiest way to think about this is that the Sun, Moon and You make a triangle and the sunlight hits the part of the moon and reflects down to You.

The Sun is lighting part of the far side of the Moon but the light is reflecting away from You.



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The sun is a lot further away than the moon, but we perceive them both as being the same distance, hence the illumination can appear paradoxical.

Edited by acey
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Hi Starjust, the best way to gain an understanding of the lunar phases is to watch the Moon for a month.

A few days after New Moon, go out at sunset and look towards the horizon where the Sun has just set. You'll see the Crescent Moon trailing a bit behind the Sun, with the sunward side illuminated. Every day, the Moon travels about 13 celestial degrees, so on the next night and every night after, you'll see that it's further and further away from the Sun, and a bit more illuminated. We call this a 'waxing' Moon.

Full Moon marks the end of the first half of the Moon's monthly journey around Earth. At this point, it's directly opposite the Sun in the sky from our perspective, and we see the full disc of the Moon bathed in sunlight. Sometimes we line up 'just right' and Earth blocks the sunlight from hitting the Moon, resulting in a lovely Lunar eclipse. This can only occur during Full Moon when Earth is between the Moon and the Sun.

Before the night of the Full Moon, the Moon was travelling 'away' from the Sun (from our perspective) after sunset. After Full Moon though, it's 'approaching' the Sun prior to sunrise, and the Moon's illuminated area decreases every night. We call this a 'waning' Moon.

During the waning Moon phases, the shadowline changes just as it did in the waxing phases, but in reverse.. always with the sunward limb being illuminated. It's monthly orbit has continually moved it 13° every 24 hours, and the Moon has now come full-circle to meet the Sun again on the day of the New Moon.

During New Moon, we sometimes line up perfectly and the Moon blocks part (or all) of the Sun, resulting in a Solar eclipse.

Hope this helps. :o

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