Friday, February 20, 2015

Venus, Mars and the Moon - February 20, 2015 - PART 3 - by Contributing Astronomy Editor, Dan Matlaga



If you were watching the western early evening skies this past week you would have seen the decreasing separation between the planets Venus and Mars (Figure 1). Minimum separation occurs Friday evening with the addition of the thin crescent moon.


The view of the western early evening sky of February 20 is illustrated in Figure 2.  Venus outshines both the moon and Mars.  Although the moon is represented in Figure 2, it does not register at the resolution of the figure, though the moon will be easily visible to the casual observer facing west.


A pair of binoculars might enhance the naked eye view.  Figure 3 is the view through a pair of 7X35 binoculars.  This is a popular size of binocular.  The first number "7" designates the magnification of the binocular. The size of the large lenses furthest from your eye while using the binocular provides the diameter of the lenses in millimeters, in this case "35" millimeters or roughly 1.5 inches.

 Note the image of the moon.  It consists of a bright crescent.  That bright crescent is the part of the moon illuminated directly by the sun, light from the sun reflected off the moon to earth.  The rest of the moon has an ashen look.  The ashen light is the result of light from the sun striking the earth, reflecting off the earth to the moon, then reflecting off the moon to the earth and into your eyes.  Not bad for the moon which reflects on average only 12% of sunlight, and is therefore a very dark object. This view of the moon has been described as: "The old moon wi' the new moon in her arms."

The scene before your eyes the evening of the 20th is somewhat of an illusion. Venus, Mars and the Moon are not that close in space.  We can illustrate this using the speed of light. A beam of light can travel 8 times around the earth in one second, yet that same beam of light takes about 8.3 minutes to travel from the sun to the earth. We say then the sun is 8.3 light minutes away from the earth. On the 20th of February the sunlight reflected off the moon takes but 1.2 seconds to reach the earth, Venus nearly 12 minutes and the reflected sunlight from Mars takes 18 minutes.

This is the third of four submissions concerning the planet Venus in the western evening sky. As you observe the Moon, Venus and Mars this evening, you might take a second to look directly east.  The brilliant star like object hovering directly above east is the planet Jupiter.  

The very close appearance of Venus and Jupiter above the western horizon the evening of June 30th will be the 4th of 4 postings.  

The separation between Jupiter and Venus is decreasing already. Look tonight to the west at Venus, then to the east at Jupiter... the June 30th event has begun!