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Thread: Moon and Sun in daytime

  1. #1
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    Moon and Sun in daytime

    So I never understand the whole thing about how during the day the sun and moon can both be visable, but at night only the moon is visa le. Now I know itís all due to the earth rotation.

    But why would earth rotation not hide the moon during the day?

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    Quote Originally Posted by Sinbad View Post
    So I never understand the whole thing about how during the day the sun and moon can both be visable, but at night only the moon is visa le. Now I know itís all due to the earth rotation.
    I think itís easy to understand if you think of it this way. The sun and moon are both going around us (from our perspective) on different but regular schedules, so half of the time they are in the sky at the same time and half of the time they are not.

    The only reason the sun is never out at night is because by definition, the night is the time when the sun isnít out. If the moon were brighter than the sun, then the moon would never be out at night.
    As above, so below

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    Quote Originally Posted by Sinbad View Post

    But why would earth rotation not hide the moon during the day?
    It does. The moon appears and disappears on a 24 hour cycle just like the sun and other objects. Itís just that it isnít always in the same place in the sky as the sun, so the time it appears and disappears is not aligned with the sun.
    As above, so below

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    Maybe this video of an orrery of the Earth, moon and sun will help you understand the orbital motions (and made with LEGO bricks no less!)

    https://youtu.be/chFF_t79qOM

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    Thank you

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    So I noticed I usually see a half or Cresent moon in the day. Is a full moon possible

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    You would not see a perfectly full moon in the day, because of the mechanics. If the moon is full, it means that the moon is on the other side of the earth from the sun. That is why it is full. You can imagine the same situation when you are talking to a person outside. If you (the earth) are standing between them and the sun, you will see their face very clearly lit, but if they are closer to the sun, then you can't see their face well (a new moon) because the sun is behind them. And when the moon is on the far side of the earth, we can see it when we are facing away from the sun (at night).
    As above, so below

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    So what is it called when you see the moon as a full circle, I know it’s not always concidered a full moon

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    Quote Originally Posted by Sinbad View Post
    So what is it called when you see the moon as a full circle, I know it’s not always concidered a full moon
    I think it's always considered to be a full moon. There may be other names, but that won't stop it being a full moon,

    Grant Hutchison

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    Quote Originally Posted by Sinbad View Post
    So what is it called when you see the moon as a full circle, I know itís not always concidered a full moon
    Technically, a Full Moon is an instant in time - when the maximal amount of the Moon is lit. Any time before or after, it is slightly off centre - a gibbous moon.
    (Same is true of a New Moon.)

    It can never be fully lit except during a lunar eclipse - when the Moon is directly opposite the Sun.

    However, in practice we consider the Full Moon to last about 3 days.

    It is often very difficult to detect by eye that the Moon is slightly less than full. One edge may look a hair dimmer than the other.


    This is not an ideal citation, but it does express my take explicitly and succinctly:


    "Technically, each phase of the Moon lasts only a brief instant.
    However, as Full Moon can last for a few days to the naked eye.
    Our celestial orb can in fact, appear to remain full for about three days."
    https://www.express.co.uk/news/scien...r-hunters-moon

  11. #11
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    BTW: you can never see a perfectly full moon from the Earth, because when it's in a position in which it would be fully illuminated as seen from the Earth, it's eclipsed by the Earth's shadow. So what we call "full moons" are actually a few degrees away from face-on illumination. This means that we can actually see a "full moon" in daylight, with the moon rising just before sunset, or setting just after sunrise.
    We can even (potentially) see a moon that is exactly opposite the sun (and therefore in eclipse) in daylight, because of atmospheric refraction. The bending of light by the atmosphere lifts the sun and moon by more than their own diameter when they approach the horizon - which means they can be exactly opposite each other on either side of the Earth, but still both above the horizon. (I doubt whether the moon would actually be visible in this sort of situation, because its eclipsed surface brightness would be overwhelmed by the brightness of the antisolar sky - but it would be there. Someone on the moon could shine a laser in our direction, for instance, and we'd be able to see it above the horizon while the sun was still in the sky on the opposite horizon.)

    Grant Hutchison

    ETA: overlap with DaveC426913

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    Quote Originally Posted by grant hutchison View Post
    BTW: you can never see a perfectly full moon from the Earth, because when it's in a position in which it would be fully illuminated as seen from the Earth, it's eclipsed by the Earth's shadow.
    Well, you still can see it, it's just dimmer than usual, and usually reddened.

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    Quote Originally Posted by DaveC426913 View Post
    Well, you still can see it, it's just dimmer than usual, and usually reddened.
    But is the thing you can see ever called a full moon?

    Grant Hutchison

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    What would it be called if not a full moon

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    Is "full moon" moment defined as Moon at antisolar point or Moon at antisolar meridian?
    Because of inclination of Moonīs orbit, Moon is over 5 degrees from ecliptic at, is the term "antinode"?

    You could see full Moon and Sun at opposition at noon, Sun low on the equatorial horizon and Moon low on the polar horizon. Or you could see full Moon and Sun at opposition at midnight, Sun low on polar horizon and Moon low on equatorial horizon.

  16. #16
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    I guess that a real "full moon" would only happen during a lunar eclipse, and then only on the single point on earth which is on the line between the closest point on the sun and the closest point on the sun.
    As above, so below

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    Quote Originally Posted by Sinbad View Post
    What would it be called if not a full moon
    When it's eclipsed, it's generally not called a full moon, but an eclipsed moon.

    Grant Hutchison

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    Sorry I mis spoke my question of the moon is a full circle during the day, is it still concidered full

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    Quote Originally Posted by Sinbad View Post
    Sorry I mis spoke my question of the moon is a full circle during the day, is it still concidered full
    Yes, for the time period of the full moon (which lasts several days), it is often possible to see the moon low in the daytime sky, and it's considered to be a full moon. Unless it's eclipsed, when we generally don't call it a full moon.

    Grant Hutchison

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    So why is it at times we can see the moon as a full circle but it is still not concidered full

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    Quote Originally Posted by Sinbad View Post
    So why is it at times we can see the moon as a full circle but it is still not concidered full
    Can you give an example where you think this applies? I can't think of a circumstance in which that is true, except during lunar eclipse.

    Grant Hutchison

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    Disc of Moon is 30 minutes diametre - 15 minutes radius.
    Cosine of 30 degrees is 0,866. Meaning that of Moon 30 degrees of sky from antisolar point has 13 minutes of radius light side, only 2 minutes wide crescent of dark side. As the Moon approaches antisolar point, already about 20 degrees from antisolar point the crescent of dark side is under 1 minute width... and dark side against dark sky in contrast to light side is invisible.

    When does Moon get the name of "full"?

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    Thank is what I was wondering also

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    Astronomically, the full moon marks an instant in any lunation, when the apparent longitude of moon and sun differ by exactly 180 degrees.
    Practically, the full moon lasts for as long as the illuminated disc appears circular to a given observer - something from two to four days.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    Astronomically, the full moon marks an instant in any lunation, when the apparent longitude of moon and sun differ by exactly 180 degrees.
    Obviously those are identical 4 times a year, at nodes and solstices, but do moon and antisolar point have to share the same longitude, or the same projection to ecliptic?

  26. #26
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    The same apparent geocentric ecliptic longitude, according to the definition used by the USNO Astronomical Almanac.

    Grant Hutchison

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    Quote Originally Posted by chornedsnorkack View Post
    Disc of Moon is 30 minutes diametre - 15 minutes radius.
    Cosine of 30 degrees is 0,866. Meaning that of Moon 30 degrees of sky from antisolar point has 13 minutes of radius light side, only 2 minutes wide crescent of dark side. As the Moon approaches antisolar point, already about 20 degrees from antisolar point the crescent of dark side is under 1 minute width... and dark side against dark sky in contrast to light side is invisible.

    When does Moon get the name of "full"?
    That is in good agreement with my personal experience. When the Moon is that close to the antisolar point, my unaided eyes cannot distinguish it from a perfect circle. A telescope will show the shadows on the edge toward the antisolar point.

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    Quote Originally Posted by Hornblower View Post
    That is in good agreement with my personal experience. When the Moon is that close to the antisolar point, my unaided eyes cannot distinguish it from a perfect circle. A telescope will show the shadows on the edge toward the antisolar point.
    Opposite?

    Grant Hutchison

  29. #29
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    Quote Originally Posted by grant hutchison View Post
    Opposite?

    Grant Hutchison
    No, the dark side is away from the Sun, and thus toward the antisolar point. I once got mixed up the same way.

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    Quote Originally Posted by Hornblower View Post
    No, the dark side is away from the Sun, and thus toward the antisolar point. I once got mixed up the same way.
    You're right.

    Grant Hutchison

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