Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Sideways Moon

  1. Dec 9, 2007 #1
    OK...this is a silly question, but it was something I was wondering about since I saw it a week or so ago.

    Let me just preface this with that I live in Michigan, USA. I do not know the co-ordinates of the area, but I assume that is enough information to answer this remedial question.

    Anyways, While out the other night I saw a half moon that was sideways. By this I mean the visible "lighted" side of the moon was the bottom half, rather than the left, or right half. It was nearly at an 180 degree angle from my vantage point. I have never seen this before at all and have lived in this area for 20+ years.

    Anything interesting about this anyone could tell me?
  2. jcsd
  3. Dec 9, 2007 #2
    Bottom half lit basically just means the sun was below you, which is fairly normal at night time really. Especially in the middle of the night in winter (as for you in the northern hemisphere).

    I guess the reason you normally see the moon lit from the side is because you're used to only watching the moon just after dusk.
  4. Dec 9, 2007 #3
    hmmm...weird. I'm pretty much a night owl so it was surprising to see it that way. First time I have which is strange since I am up all night a lot and outside.

    Figured it was a pretty logical answer like that. Just wanted to make sure there wasn't something interesting I may have been missing out on so I asked :). Thanks.
  5. Dec 10, 2007 #4


    User Avatar

    Staff: Mentor

    The sky rotates around polaris, so every object in it will have rotated 180 degrees or so when going from the east horizon to the west horizon. Note though, that depending on the season, the moon will rise further north (in summer) or further south (in winter) and be in the sky different amounts of time, so the amount of rotation will vary by how long it is in the sky.
  6. Mar 12, 2008 #5
    moon watching CA

    I noticed the moon on the 9th also. Which is how i came across this. I feel like the other person in the way that i am surprised that i have never noticed it before. So it appears like this around dusk everyday?
  7. Mar 12, 2008 #6
    Wait... i saw the 9th on the previous date stamp and assumed this was a recent post. Nevermind...jump to 3 months! Now i can see that it must occur often!
  8. Mar 12, 2008 #7


    User Avatar

    Staff: Mentor

    Not every day, every month.
  9. Mar 12, 2008 #8


    User Avatar
    Science Advisor
    Homework Helper

    Hi Tomato! Welcome to PF! :smile:

    A half moon means that the sun is 6 hours away from the moon.

    If the sun is directly below the moon, that means the time is half-way between sunset and moonset (or moonrise and sunrise).

    So this will be seen once a month, three hours after sunset, and two weeks later, three hours before sunrise! :smile:
  10. Mar 12, 2008 #9


    User Avatar

    Staff: Mentor

    Well, no. The ecliptic is tilted, but it never flips over itself, so the only time the sun and moon are at the same azmuth is during a new moon. 6 hours away just means they are 90 degrees apart in azmuth and that is only loosely connected to when they will rise or set. What it does mean is that if the sun is due west right now, the moon is due south right now and 6 hours from now, the sun will be due north and the moon due west. But depending on the tilt of the ecliptic for your season and location, the sun, the moon, both, or neither may be up at either of those times.

    For example, first quarter is this Saturday. 3 hours or so after sunset (9:35), the moon is at an azmuth of 251 degrees and the sun at 294. The sun and moon both trace out the entire azmuth circle in a day, with the sun chasing the moon for half the month and speeding away for the other half.

    What people are noting is the tilt of the ecliptic. Once a day in the middle latitudes, the ecliptic is near vertical on each side of the sky, with the times dependent on the season. So when the moon is on it in that part of the sky at that time, it is laying on its side. And this can happen in any phase and happens every month at a different time and in a different phase. Here's the view last night (ecliptic near vertical) and August 5 (nowhere close to vertical), both times a little less than 5 days past new.

    For yesterday, note that the moon is a little above the ecliptic, which futher steepens the angle.

    Attached Files:

    Last edited: Mar 12, 2008
  11. Mar 13, 2008 #10
    Originally Posted by tiny-tim
    A half moon means that the sun is 6 hours away from the moon.

    I dont understand it. Could you explain this to me. I am a bit new to amateur astronomy and am used to watching night sky with naked eyes.
    I've been watching the night sky for about two years but I didnt ever took notice of the moons stances.
    This would be a cool thing to observe.
  12. Mar 13, 2008 #11


    User Avatar
    Science Advisor
    Homework Helper

    Hi anonymoussome! :smile:

    The moon and the sun are basically on the same circle in the sky, so every day they follow each other round that circle, with the sun gradually catching up and overtaking once a month.

    (Technically, they're on two different circles, very close together, with the plane of one tilted about 5ยบ with respect to the other.)

    At full moon, the moon is on the opposite side of the circle to the sun, so it rises and sets 12 hours later (and earlier!). And, since the earth (that's us) is always in the middle of the circle, the sun "sees" the same side of the moon as we do - in other words, we see the whole of the sunlit side, which is why we call it a full moon.

    At new moon, the moon is very close to the sun on the circle, and it rises and sets about the same time as the sun does.

    At first quarter and third quarter (which, confusingly, is when the moon looks half full!), the moon is a quarter of the way round the circle, and so rises or sets a quarter of a day - that is, 6 hours - before or after the sun does. :smile:

    Everything in the solar system moves anti-clockwise (as viewed from the northern hemisphere), so if you face south, you will notice that the moon is getting further away from the sun to the left as the month progresses.
  13. Mar 13, 2008 #12


    User Avatar

    Staff: Mentor

    Again, this is not generally true. It depends on the season, obviously, since the sun and moon do not stay in the sky for exactly 12 hours everywhere, every day. In fact, the only time that both the sun and moon will be in the sky for almost exactly 12 hours is once every 29 years, on each equinox, when it's a new moon.
    Also still not true, as I described in the post above.

    If you have a piece of planetarium software, I highly recommend you play with it to prove this to yourself. And I can post more screenshots if you want.
    Last edited: Mar 13, 2008
  14. Jan 14, 2011 #13
    I actually thought i was the only one, i saw the moon exactly how you desribed it and I live in New York. So I thought it was just one of those things that happen and they are amazing. I just couldnt think of a scientific explanation for it. I dont even think the scientists and astronomers noticed this natural phenomonon. But its good to know that someone else saw it too. :)
  15. Jan 14, 2011 #14
    See that makes sense but in all the world its NEVER happened before so it cant be just something like that.
  16. Jan 14, 2011 #15
    How is the sun under the moon?
  17. Jan 24, 2011 #16

    My first post here !

    I am not familiar with astronomy at all, but, I just notice that the noon is indeed "sideways". The moon is dark on the top when it comes up ( 11:22pm ) and then in the morning it is dark on the left bottom side. We are on the waning gibbous phase of the noon. Oh, yes, I live in Miami, Fl.

    I have read the previous posts, but I really do not understand--so if someone could put it in very elementary terms I would appreciate it.

    Thank you in advance !
  18. Jan 24, 2011 #17


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The illuminated side always faces the sun, right? So relative to us, the moon is illuminated from where the sun resides 93 million miles away. At dusk it is somewhat below us and in the day, somewhat above us.

    At night the waxing or waning moon is lit from the bottom when the moon is near the horizons (yes, both of them). So the waxing moon you see at dusk in the west is a crescent that is lit from below (the horizon) and the waning moon you see at night in the east (at moonrise) is likewise lit from below. As the waning moon moves across the sky it changes its relative orientation... top becomes bottom and bottom becomes top. Left becomes right and right becomes left.
  19. Jan 24, 2011 #18
    I think I (she's) got it--as in M Fair Lady. While I was driving, I was thinking and I actually figured it out.

    Then I read the lastest post and yes--I see the light (ha ha )

    Thanks you all very much:wink:
  20. Jan 25, 2011 #19
    I noticed a sideways moon a few months ago and it puzzled me for a few days, but I never bothered to ask someone about. Glad to see it popped up here
  21. Jan 26, 2011 #20
    all this scientific stuff sounds great but you arent answering the question. because i witnessed this same event tonight in Oklahoma and it was a very bright tint of yellow and horizontal rather than vertical. if it was just a monthly thing that happened i would have seen it before now because i am out at all hours of the night every night. and also if it happened often why isnt it included on ANY science based moon phase charts? there is no explanation on any website that is pure fact. so it leads me to believe that there is something more to this.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook