Mercury in Motion

   This week my students worked on a lab involving the planet Mercury. Part of the lab was to determine the dates for the next elongations of Mercury and also the degrees of elongation. As they worked on their lab they were able to observe Mercury (using software) as it moved from eastern elongation last month through inferior conjunction and then to western elongation next month. I was so pleased when some of them commented that Mercury was moving in retrograde motion as it traveled through inferior conjunction, October 16th, as the animated graphic is showing.
   In the animated graphic I have added the ecliptic (green line) to show the relationship between Mercury’s orbit and the ecliptic. Mercury is inclined 7o from the plane of the ecliptic and as you can see it is below the ecliptic, with the 4th of October as its maximum separation from the ecliptic.

Click on graphic to see it full size.

Click on graphic to see it full size.

   Currently Mercury is still setting after the Sun sets but that will not be for much longer. As the students noticed, when you combine Mercury’s orbital speed (which right now is toward the west and the Sun) with the apparent motion of the Sun toward the east the two close in on each other rather quickly – at least compared to when Mercury is moving toward the east after western elongation. Mercury moves at approximately 4o per day compared with the Sun’s apparent motion of approximately 1o per day.
Caution: Objects viewed with an optical aid are further than they appear.
   Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

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