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My wife and I were watching a TV show last night, which takes place in Salem, Massachusetts circa 1693. One character went to a particular place in the woods at a particular time to be exactly under Saturn. I told my wife that Saturn could never be exactly above such a Northern latitude.

What is the Northernmost Latitude that Saturn could possibly be above? By "above", I mean if you draw a line through the center of the Earth and through Saturn, the point on the Earth's surface that line passes through.

Also, how often would it reach that point? Would it be every few years, or every few thousand years?

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  • $\begingroup$ That was my reasoning too. Except for the 29 years thing, I didn't know that. And I also am not sure whether there was something I missed. $\endgroup$ – David Dubois Mar 21 '15 at 19:35
  • $\begingroup$ @WayfaringStranger Why not make it an answer? $\endgroup$ – ProfRob Mar 22 '15 at 17:13
  • $\begingroup$ In addition to @WayfaringStranger's great answer and your last "how often would it reach that point?": note that whenever Saturn is positioned on the dark side of the earth, there is every night a spot (actually a line!) on earth where Saturn in straight above. $\endgroup$ – agtoever Mar 23 '15 at 6:33
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Salem is at 42.5° north. At the equator, the position of the sun varies by 23.5 degrees on either side of the vertical over the course of each year, due to earth's 23.5° axial tilt with respect to the ecliptic.

Saturn, however is not on the ecliptic. It departs by about 2.5° over the course of its 29.45 year, 9.5 AU orbit. So once every 29.5 years Saturn's angle to the ecliptic will add 2.5° to the max latitude the planet can be seen overhead. That adds up to 26° as the max latitude Saturn can be seen directly overhead.

There's one more factor to account for though. Earth being 1AU out from the sun, it's only 8.5 AU from Saturn at close approach. That will add another 0.2° (by trig) to the max latitude at that time of year when earth is nearest Saturn. That gets us to the possiblity of Saturn appearing directly overhead at up to latitude 26.2°: Miami, Jaipur, or Dubai, not Salem

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To add to the other answers, if you were in a place where Saturn could be directly above, make sure you get there just over an hour and a half early.

Now; do you need to be there when Saturn is directly overhead or when the light from Saturn is directly overhead?

The closest Saturn gets to earth is approximately 1,200,000,000 km (750,000,000 miles), light takes 1 second to travel 300,000 km therefore the light from Saturn would take a minimum of 4000 light seconds (1,200,000,000 / 300,000 = 4000) 4000 seconds = 1.11 hours. So you are seeing Saturn where it was just over an hour before. This could increase to over 1 1/2 hours depending on the distance between Earth and Saturn at the time (max approx 1,700,000,000 km).

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