MadSci Network: Astronomy

Re: do we see the same view of the moon from the north and south hemispheres

Date: Mon Jun 19 13:09:18 2006
Posted By: John Link, Senior Staff Physicist
Area of science: Astronomy
ID: 1150562315.As

The question is "Do we see the same view of the moon from the north and south hemispheres?" Technically the answer is "no", but practically the answer is "yes"!

There are several differences between the northern and southern hemispheres that are pertinent to this answer:

  1. When looking at the moon from the northern hemisphere, "left" is not the same as "left" looking from the southern hemisphere. That is, if you stand in the northern hemisphere facing the Earth's equator you would say that the Sun "goes" "from left to right" during the day, but if you stand in the southern hemisphere facing the Earth's equator you would say that the Sun "goes" "from right to left." In both cases the Sun "moves" from the eastern part of the sky to the western part of the sky, of course. In the same way the Moon "goes" "from left to right" across the sky during one evening if you view it from the northern hemisphere, but "from right to left" from the southern hemisphere. (During its orbit around the Earth the Moon goes "from right to left" from the viewpoint of the northern hemisphere, but our perception during one evening is dominated by the Earth's rotation!) The Moon is "upside-down" in the southern hemisphere with respect to the northern hemisphere. (Of course if you live in the southern hemisphere you would say that the Moon is "upside-down" from the northern hemisphere!!)

  2. The actual face of the Moon that you see depends on your latitude on the Earth. (There are also some second-order effects such as the eccentricity of the Moon's orbit, but I won't discuss those here because the effect is the same no matter if you are in the northern or southern hemisphere.) Look at the following picture as you read along, but realize that the distance from Earth to Moon is not to scale

    Let's say that someone standing on the Earth's equator has the "proper view" of the Moon, looking straight down on its equator. If you are in the northern hemisphere at latitude 41 degrees, which is roughly the latitude of New York, NY, you are about 4180 km closer to the north pole than at the equator (measured along the Earth's rotational axis)(4180 km = 6370 km * sin(41o), where 6370 km is roughly the radius of the Earth). Since the distance between the Earth and the Moon averages 385000 km, you would be looking "straight down" on a part of the face of the Moon that is about 0.6o (This angle is the arcsin of 4180 km / 385000 km) north from the Moon's equator. At the same latitude in the southern hemisphere (roughly the latitude of Tasmania) you would be looking at the Moon's face about 0.6o south of the Moon's equator. A difference in viewing angle of 1.2o is unnoticeable by the average onlooker! In fact, your view of the Moon's face shifts about 1.4o (assuming the 41o latitude) during its "trip" across your sky from the eastern horizon to the western horizon in one evening, and I'm betting you have never noticed the difference!

So, the Moon looks mostly the same from northern and southern hemispheres of the Earth except that one view is "upside-down" with respect to the other.

John Link
MadSci Physicist

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