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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Beginner moonrise and set question
From: Trevor Kenchington
Date: 2004 Sep 22, 23:55 +0000
From: Trevor Kenchington
Date: 2004 Sep 22, 23:55 +0000
Bill, Your calculation of the time of moon rise is in Local Mean Time (LMT) for longitude 86? 57'W. Your watch, however, is set to EST, which is to say Z+5 or LMT for longitude 75?W. The required adjustment of +48 minutes is the conversion from your LMT to EST. If you had set your watch to Central Standard Time, the adjustment would be -12 minutes instead. In other words, this is a matter of the definition of time measurement, not one of the apparent movement of astronomic bodies. Since we define 24 hours as the length of a mean solar day, the same arc-to-time conversion works for both calibrating time among watches set for different longitudes and for the change in the Sun's GHA with time. But the adjustment you need to make is the same regardless of what object you sight since it is an adjustment between two terrestrial measures of the same instant in time. As to: > A more complex question--at least to me: > > Regarding the Earth, Moon, and Sun I recently read (if I correctly remember > and understood it) that the Earth and Moon actually rotate--in relation to > each other, about a point 800 miles from the Earth's surface. > > To draw a 2D analogy, if a 300 lb. person and a 30 lb. person were on a > see-saw, the fulcrum would have to be much closer to the large person to > achieve balance. If we were to spin the pair around the fulcrum, in > relationship to a nearby post, each person would get nearer then farther > from the post with each revolution, the 30 lb person more so because of the > greater distance from the pivot point. > > My question: Ignoring precession and other wobbles, does the Earth's axis > go smoothly around its elliptical orbit of the Sun; or do the Earth/Moon > pair spin around some point 800 miles below the Earth's surface between the > Moon and Earth's centers, held together by gravitational attraction (like > the pair above held together by the see-saw) with each getting nearer and > farther from the Sun as they go about their journey? I should really leave that to others better qualified to answer it but I think that it is the centre of mass of the Earth/Moon system (lying, I think, around 1060 miles or 1700 km below the surface of the Earth) which follows a steady elliptical orbit around the Sun (ignoring other wobbles). The Earth's axis then moves towards and away from the Sun as the Earth/Moon system makes its monthly spin around the common centre. However, if you are in a reasonably low latitude, your distance from the Sun increases and decreases more with each daily rotation of your position around the Earth's axis than it does with each monthly rotation around the Earth/Moon centre of mass. Trevor Kenchington -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus