Monday, January 19, 2015

"Close Approach " of Venus and Mercury PART 2, by Contributing Astronomy Editor, Dan Matlaga

                                    January 20-22, 2015

 
FIGURE 01
Let's examine Figure 01, depicting the southwest backyard sky at about 6:15 p.m. on January 20, 2015. This is a time when a half-dozen stars are visible in your evening sky. Features include the positions of the Moon, Mercury, Venus and Mars.

A line drawn from the "S" which stands for South and to the "W" which stands for West would be the horizon if you were standing on a boat in the middle of the ocean. 

The Moon in our setting is just above the South-West horizon.  It is an eleven hour moon.  At 7 a.m. this morning the moon and sun were on the same north south line. The moon travels around the earth such it moves its own diameter each hour, or as viewed in the sky, half-degree each hour. At the time of our diagram, the moon has separated from the sun by 5.5 degrees.

Technically above the horizon, practically it would be very difficult to observe the 11-hour moon this close to the horizon, even though you were trying to observe it atop a large sand dune in the middle of the Sahara.  Only 0.4 percent of the moon is illuminated.  Figure 02 illustrates how little this is.
FIGURE 2



The illuminated crescent part of the moon does not present a continued light line, but is broken up in places due to the moon's crater walls seen edge on.

The brightest image in our sky is the planet Venus. It is the anchor in this our second posting of four; since our last posting of January 10th, when Venus and Mercury were very close in the sky.  Now, just ten days later, Mercury has peeled off to the west and below Venus. 

Mars is above Venus.  It will be interesting to watch from one evening to the next as their separation decreases.


                                    January 21, 2015


Just 24 hours later, we note from comparisons of last evening sky to this, the increasing separation between Venus and Mercury. (Fig. 03)
FIGURE 3
Last evening the Moon was 11 hours old. This evening it is 11 hours old plus 24, or 35 hours old.  It has increased from 0.4 percent illumination last evening to 1.3 percent this evening. (Fig.04)
FIGURE 4
This evening marks the first evening when the crescent moon is an easy target for the backyard observer.  Finding the moon's position is aided by its close proximity to the planet Venus. Two nights and three days ago the moon was an equally visible object in the morning sky immediately before sunrise.  Two nights and three days equal 72 hours. It took the moon 72 hours to pass from the morning sky to the evening sky. Half of 72 is 36.  We have a 35 hour moon this evening in good agreement of a standard two nights and three days traditional description of the dark of the moon, when the moon is not visible to the unaided eye.  

The Greek goddess of the Moon was Artemis. As the goddess of transitions she was a goddess of birth and death.  The crescent moon mythically died two nights and three days ago but now reborn as a crescent in this evening sky. A Greek word to describe a thin crescent moon is the word "Cynthia," abbreviated as "Cindy." Parents in search of a name for their newborn find Cynthia listed as another name for the Moon goddess Artemis.  This is a translation I would expect from a high school guidance counselor or worse yet, from a gym coach. An appropriate sense of the name Cynthia is: "She who hunts among the clouds."

The separation of Venus and Mars continues to decline.


                               January 22, 2015


This is the evening sky at about 6:20. (Fig. 05)
FIGURE 5
Note the position of Mercury. It is getting very low and by month's end will rise and set at the same time as the sun.

In just 24 hours the moon has changed its position from the west of Venus to the west of Mars. If we could pace the apparent distance the moon traveled from last evening to this evening, multiply it by 29.5 times, we would have marked a great circle around the sky. We would find the moon back to the same phase as tonight. This number is near the 30 day cycle of the month.  The month does indeed get its length from this, the Synodic cycle of the moon.

The moon is 9 percent sunlit. (Fig. 06)
FIGURE 6
I refer to this phase of the moon as the Romantic Moon, because it will take the moon 2.5 hours to set; more than enough time to accomplish the tasks at hand.