Robin Rosendale, age 11, of Saratoga, California, for her question:
When can Venus be seen without a telescope?
Early astronomers named the bright Morning Star "Lucifer" and the golden Evening Star "Hesperus." Later they realized that the two were the same planet traveling on opposite sides of the sky. They named it Venus and all this planet charting was done ages before the telescope was invented.
At its brightest, Venus outshines every star in the sky. Tonight, as the sun sets, it will be the first bright eye to open in the starry sky. It will appear in the west in the role of the golden Evening Star. You do not need a telescope to see it for the next few weeks. But late in August, Venus passes between us and the sun with its night side facing. the earth. At that time, you may or may not be able to spot it with a good strong telescope. Later, the golden planet will appear in the eastern sky at dawn in the role of the Morning Star. But early in November it will pass behind the sun and you could not see it. with the best of telescopes.
Venus is closer to the sun than we are and its smaller orbit is almost level with the earth's orbit. As it orbits the sun, we see daylight and darkness creep across its surface and Venus goes through phases like our changeable moon. However, from our point of view on the earth, Venus takes 584 days to go through the four phases the moon completes in a lunar month. During that time, Venus seems to swing like a pendulum from one side of the sun to the other.
The two sister. planets orbit eastward around the sun. But Venus with its smaller, inside track, .is forever catching and passing the earth. This is why its position in relation to the sun changes constantly. On June 21, Venus reached the limit of its eastward swing. Then, as the Evening Star in the western sky, its distance from the sun was 22 1/2 times the width of the full moon. It is now moving westward from this point; closer to the sun.
Venus is now at its~brightest and you might expect the sun to be shining full on the side facing towards us. But if you use a telescope, even a small one, you will be surprised to see that a little less than half of the round planet is in daylight. In this position, Venus is in the half phase. If you watch night by night with a telescope, you will see it shrink to a quarter phase and then to a thin sliver. Venus is catching up to us to pass between us and the sun. As it gets closer to the sun, it gets slimmer because moxe.of its night side is turned towards us and for awhile it disappears altogether.
It reappears on the opposite side of the sun in the role of the Morning Star. In early November it will be the last bright eye in the starry sky to blink off at sunrise. Through a telescope you see that Venus has grown to a half phase again. Bit by bit,it reaches the three quarter phase. But it does not get brighter because it is moving away from: us. As it reaches the full phase, it hides out of sight behind the sun.
The average distance between us and Venus is 26 million miles. The two planets are this far apart when Venus passes between us and the sun. But when it moves around to the other side of its orbit, we are separated by 160 million miles. Distant objects seem to shriek and grow dimmer. This happens to golden Venus as it moves away from us. When it is on the far side of the sun, its face is 2 1/2 times smaller. Even though its full face shins in sunlight, it is dimmer than the half and quarter faces we see when Venus plays the role of Morning and Evening Star.