Friday, November 28, 2008
Sunday night (November 30), right after sunset, take a few minutes to view the new crescent Moon approaching Venus and Jupiter in the southwestern sky.
Use your fist held at arm's length to measure the angular distance from the Moon up to Venus (the brighter and lower of the 2 planets)- your fist at arm's length is 10 degrees wide. Here's a Stellarium view of what you'll see - the red line is the plane of the ecliptic.
On the evening of 12/1 (Monday night), the Moon will have moved eastward to a position just to the east of Venus. Using your fist again, estimate how far past Venus the Moon is, and add that measurement to last night's to estimate how far eastward among the stars the Moon has moved in the last 24 hours.
Divide that number into 360 degrees to calculate the approximate number of days it takes the Moon to go through a cycle of phases (called the synodic period*) I'll post my results here next week.
*Here's a diagram that explains the sidereal (one revolution) and synodic (one cycle of the phases) of the Moon. If you measured the Moon's eastward progress against the stars, you could calculate the sidereal period. But since Venus is moving eastward against the stars, too (with the Sun), you're really measuring the synodic period.
Now take a look at this LA Times article, and pay close attention to the image. Can you tell what's wrong? Can you explain how it might have happened?