The Rocking Moon

   On Monday September 15th the Moon will show its maximum libration. Lunar libration is a tilting motion of the Moon as it revolves around the Sun with the Earth. In a way like how the Earth’s axial tilt gives rise to seasons as the Earth “leans toward the Sun in northern hemisphere summer, and then is tilted away from the Sun for northern hemisphere winter”. From the Sun this would look like the Earth is nodding back and forth – tilting towards and then away from the Sun.
   Our Moon has an orbit that is tilted about 6o away from the Earth’s orbit, and the Moon has an axial tilt of about 1.5o. So the Moon will also have this apparent motion of tilting back and forth and from side to side every day, as well as every lunar cycle.
   There are three ways to look at libration (seriously – three ways to look). However all three combine to give a nausea inducing motion if sped up!
   Lunar phases have been ‘turned off’ for this simulation.

   Lunar phases are shown for this simulation.

   Libration in Latitude has the Moon nodding forward and backward allowing one to actually see more than 50% of the Moon. During this libration more of either the Moon’s north or south pole is more visible – up to 59%. Libration in longitude is an effect caused by the elliptically shaped lunar orbit. This causes the Moon to oscillate back and forth appearing larger or smaller. The third type of libration is Daily or Diurnal Libration. This is a left to right type of motion caused from the different viewing perspectives between when the Moon rises and when it sets. The setting Moon is approximately 4,000 miles (6,437 km) further from an observer then at moonrise. This difference is enough to give the Moon a left to right oscillation.

Click on graphic to see it full size.

Click on graphic to see it full size.

   The waxing gibbous Moon rises around midnight local time and is above the southern horizon at sunrise.

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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