The discussion revolves around the observation of the Moon from a location near the equator, specifically addressing how the orientation and visibility of the Moon's features relate to the observer's position and the Moon's orbital characteristics. Participants explore concepts related to lunar phases, angles of visibility, and the effects of the Moon's tilt and libration.
Participants express differing views on the visibility of lunar features and the implications of the Moon's tilt and orbital characteristics. There is no consensus on the exact nature of the observations or the best way to conceptualize them, indicating ongoing debate and exploration of the topic.
Participants note that the discussion is influenced by various factors such as latitude, the Moon's axial tilt, and the angles involved in the Earth-Moon-Sun system, which complicate straightforward interpretations of lunar visibility.
You can never see both poles at the same time. There is a significant shortfall when the object you are looking at is as large and near as the Moon.lifeonmercury said:I'm standing at a location near the equator and see a full moon in the sky overhead. From my vantage point, the top point of the Moon is (approximately) at the Lunar equator and the Lunar poles are 90 degrees away at the sides. Is this correct?
phyzguy said:I think it's much more complicated than you say. It depends where on the ecliptic the moon is, since this will determine the angle between the ecliptic and the horizon. Also, the axis of the moon's orbit is tipped 5 degrees relative to the ecliptic plane, and the Earth's axis of rotation is tipped 23 degrees relative to the ecliptic plane. Why don't you try sketching it out when the moon is on the celestial equator, at it's highest point above the equator, and at its lowest point below the equator?
That effect is smaller than 1% for a perfect sphere: You can see more than 49% of that idealized sphere. Mountains etc. lead to additional surface area you cannot see, but it also leads to points beyond the idealized horizon that are visible.sophiecentaur said:You can never see both poles at the same time. There is a significant shortfall when the object you are looking at is as large and near as the Moon.
A bit of trigonometry would tell you just how much is invisible.
Because of libration, we see much more than half of the Moon's surface so that 1% is not very relevant.mfb said:That effect is smaller than 1% for a perfect sphere: You can see more than 49% of that idealized sphere. Mountains etc. lead to additional surface area you cannot see, but it also leads to points beyond the idealized horizon that are visible.
Have a look at the diagram http://ccar.colorado.edu/asen5050/projects/projects_2011/lestishen/extension.html. That shows the angles involved. The sun lies in the plane of the ecliptic and none of the angles of the Earth's rotation, the Moon's rotation or the Moon's orbit are zero. Also, your view from the Earth will be affected by your latitude. A simple model of the system will not show you exactly what you will see.lifeonmercury said:Still having trouble grasping this concept unfortunately. When I see a half-moon in the sky divided diagonally (the dividing mark between the light and dark halves is not straight up and down), does that mean the lunar poles are approximately at the two ends where dark and light converge?
sophiecentaur said:The lunar axis will appear to be (approximately) horizontal when viewed from the Equator and (approximately) vertical when viewed from the poles.
Google Images has pages and pages of pictures of what you want. Search terms like tilt of the moon's orbit etc. Very entertaining.lifeonmercury said:Thanks. This fully answers my question. I will have a look at the diagram anyway.