Venus: Goddess of Love and a Hell of a Planet

© 1999 by Robert C. Moler

The second bright planet visible this year is Venus. The first was Mars. Since May 1st the earth has been pulling away from it.. Venus is approaching the earth, and has been since October 30th of last year when it was in superior conjunction with the sun. Venus will be at its closest and invisible at its inferior conjunction with the sun August 20th.

To clear up the terminology, a conjunction is when two solar system objects appear closest together. Officially its when they have the same right ascension, the celestial analog to earthly longitude. A superior conjunction is when a planet is on the other side of the sun. And an inferior conjunction is when a planet is between the earth and the sun. Venus and Mercury are called inferior planets because they lie in orbits inside the earth's orbit. Of course superior planets lie outside the earth's orbit.

Since Venus' orbit lies within the earth's orbit it can never stray far from the sun. And unlike Mars, Venus cannot be seen when it is closest to the earth because it would be at inferior conjunction with the sun and lost in the sun's glare. There is one exception, a transit when Venus appears against the face of the sun. Venus spends most of its time in the evening sky slowly moving away from the sun. On June 11th Venus will have its greatest separation from the sun, called greatest eastern elongation of 46 degrees. That's nearly seven and a half months since superior conjunction. From greatest eastern elongation to inferior conjunction is just a bit more than 2 months.

During the period from superior conjunction Venus changed from a tiny full disk only 10" (seconds of arc) in diameter to a half illuminated (quarter moon) disk 24" in diameter at greatest elongation. But in the next two months the apparent diameter will grow to 48" by inferior conjunction. That's larger than the apparent diameter of Jupiter at its closest and the phase will diminish to a thin crescent. Within a month of inferior conjunction Venus' thin crescent is even visible in a pair of binoculars.

Back in really ancient times, the morning appearances of Venus and Mercury were thought to be different planets. However after a while someone figured out that when the bright Morning Star reigned the Evening Star was absent. And when the Evening Star disappeared close to the sun, the Morning Star appeared a few weeks later.

Venus also has the reputation of being visible in the daytime. It helps if Venus is at greatest brilliancy which occurs 36 days before and after inferior conjunction. That's July 15th at magnitude -4.3. It also helps to look near sunset if Venus is in the evening sky, or near sunrise if Venus is in the morning sky. Make sure you are shielded from direct sunlight and the sky is crystal clear. It takes good eyes and luck to latch on to Venus in the vast blue sky.

In a telescope Venus is dazzling and featureless except for its phases. Venus is surrounded by vast clouds of sulfuric acid vapor. The cloud features seen in photographs are visible in ultraviolet. A violet filter like Wratten 47 may help see some cloud features. I've never seen any.

One observational oddity is the difference between greatest elongation and dichotomy (the appearance of Venus being exactly half illuminated). At greatest elongation of a planet the sight line from the earth to the planet is a tangent to the planet's orbit, so the terminator, the line that separates day from night should exactly bisect the planet. But it doesn't actually appear so. As the phase is decreasing, dichotomy should occur about a week after greatest elongation, or about June 18th.

This evening apparition of Venus will end this summer, at it becomes a larger and larger but thinner crescent in telescopes, a bad time to spot planets close to the sun. Summertime is bad because the ecliptic the apparent path of the sun, and near where all the planets lie is tilted low to the south. In the diagram on this page, from my Ephemeris web site, Venus is seen at sunset from November last year to August this year. The celestial grid lines are 15 degrees apart. Note that Venus' greatest height in the evening sky occurred in mid May, while actual greatest elongation will occur in mid June. That's because the sun is sliding farther to the north on the horizon.

Venus first became noticeable in its current evening apparition about January 1st. So according to the diagram at left we would expect that the planet will be hard to spot again in late July when it reaches about 10 degrees altitude at sunset. As can be seen by the spacing of the dots in Venus' path, its passage from the evening sky will be rapid, as will it's sudden appearance in the Morning sky in early September. Perhaps it will being another UFO report from Mayfield.


Questions? Send Email to me at bob@bjmoler.org

Uploaded: 12/30/99