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

Moons shadow-line and sun

  1. Apr 30, 2012 #1
    So I'm watching the sky at daytime, and i see the moon. The shadow on the surface of the moon goes in a straight line. I therefore conclude that the angle of MeMoonSun must be 90 degrees. But a peculiar thing shows; The shadow line on the moon is not perpendicular to the straight line between the sun and the moon! This is strange...

    considering a basket ball and a flashlight, i cannot get the same result.

    Why is this? Is it because of General relativity?
     
  2. jcsd
  3. Apr 30, 2012 #2

    The shadow line is a circle on the surface of a sphere. When viewed from within the plane of this circle it looks straight (half-moon). This circle is always in a plane perpendicular to the line between the moon and the son. Why don't you think it's perpendicular?
     
  4. Apr 30, 2012 #3

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    How far out is your measurement? Could it be because you are using the time zone time and not local local time?
     
  5. May 1, 2012 #4
    Anyone who spends time outdoors will have noticed this - the Moon's shadow looks wrong for the apparent position of the Sun. This is very noticeable at Sunset and Sunrise when the Moon is much higher in the sky. The discrepancy is caused mostly by the refraction of light through the density gradient of the atmosphere.

    The atmosphere bends light as the light goes through the increasing density of the air at different altitudes. This effect is much greater than you might imagine. I was at a concert where they turned on large lasers directed up into the night sky at about a 45 degree angle to the horizon. When the beams came out they bent down to the horizon... the beams were surprisingly visibly curved.
    This is why if you time the Sun across the sky it seems to rise in the morning and set in the evening at a slower pace across the sky than at midday. When you think you are seeing the Sun at low sunset, it is geometrically already below the horizon. The air is bending the light around the edge of the Earth to continue showing you the Sun. Same thing at Sunrise, you see the Sun's image as light bending around the Earth before its true geometrical line of sight is above the horizon.

    You are also seeing the Moon from about one second ago, you are seeing the Sun from about eight minutes ago, as is the Moon and its shadow. This is a small effect. The main one is the refraction of the atmosphere.

    So basically, you are seeing the Sun in the wrong place. The Sun's correct geometric position is on a line perpendicular to the edge of the Moon's shadow... with some small error due to the shadow itself representing the Sun's position from eight minutes prior.
     
  6. May 1, 2012 #5

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Does that really make a difference? You see one interval of the Sun's emitted light at the same time as it arrives at the Moon (more or less) There is only a second's difference in arrival time of the sunlight and the reflected light from the Moon. The eight minutes of delay is not relevant because there is always some light available just over the Horizon (It doesn't arrive as a 'flash'). It could have come from twice the distance and the effect would still be the same. And why would the arrival time difference from the Moon make a difference? Also, your proposed effect would work one way when the Moon is overhead at sunrise and another way, two weeks later, when it's overhead at sunset.

    The refraction explanation is the right one to go for.
     
  7. May 1, 2012 #6
    I just stepped out and that very question began sneaking its way into my thinking - that the eight minutes would not matter. I resolved to edit my post when I returned, but was glad to see you caught it already.
     
  8. May 2, 2012 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    This transit time thing can be confusing. It's relevant for things like observations of the orbits of the moons of Jupiter. Before the advent of good chronometers for navigation, I believe they had tables of the transits of those moons to give mariners a time reference. Thank god for GPS, eh?
     
  9. May 2, 2012 #8

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    The question was asked here:
    http://astronomy.stackexchange.com/questions/1750/sun-moon-earth-anomaly

    They suggest it is a perception error and you should use a yard-stick as a visual help to check if there really is a significant offset. A small offset can be explained by refraction but that amount would hardly be perceivable without visual helps.

    Reposting the answer with references because the other site will shut down soon
     
    Last edited: May 2, 2012
  10. May 2, 2012 #9

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    This is suggesting that the apparently straight terminator is not actually straight. I could believe that easily. That probably solves our problem.
    Brill!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook