A couple of the planets in our solar system have phases similar to those we see on our own moon. These planetary phases can be observed from the earth generally with a telescope. These two planets are the two innermost to the sun.
Of course these are Mercury and Venus.
Throughout the months of April and May, the planet Venus has shown the transitions between its half-illuminated last quarter phase, and start to show itself as a waning crescent in our sky.
Venus, though generally very bright — in fact the third brightest object in the heavens after the sun and moon — becomes larger to our eyes and remains at its brightest, some negative 4.7 magnitude in its most slender of a crescent apparition.
It is said that Galileo Galilei in 1610 first observed these phases. So what causes these Venusian phases? We know that the orbit of Venus around the sun is 224.7 earth days, this is inside the earth’s orbit. When Venus's orbit takes the planet opposite the sun, Venus appears to be full.
Keeping in mind that when Venus's orbit brings it between the earth and the sun, the planet becomes a very slender crescent. But being closer to the earth, Venus, although slender, appears larger.
The world of Venus was once thought to be a balmy paradise, earth’s sister — named for the Roman goddess of love, beauty, victory and prosperity. We know it is nothing like what we had thought. Venus suffers from a runaway greenhouse effect due to its thick covering of clouds. Rain of sulfuric acid is common there, as well as a crushing atmosphere. Early Russian spacecraft sent to explore Venus were crushed like old tin cans upon arrival due to the weight of the atmosphere. And with a surface temperature of 863.6 degrees F, what was once thought to be an earth-like paradise, we now know it is the one place in our solar system that we humans think to be most like hell.
Let the thick cloud cover on Venus and the horrible greenhouse effect serve as a warning to us humans ... we may be looking at earth’s distant future if we do not keep things under control.
The interior of Venus is a metallic iron core. Under thick clouds of sulfuric acid lies a rocky surface, but not pockmarked with craters like other planetary worlds.
Although sometimes referred to as “Earth’s Sister" because of their similarities in size, orbit and gravity, Venus is a very different world, one that greets us as the brilliant morning or evening star.
Poems have been written about Venus; Edgar Allen Poe even mentioned it in the poem “Ulalume."
It is said that the planet Venus can be so bright that it will cast a shadow on the ground on moonless dark nights. And that people with exceptional vision can make out the crescent shape of Venus when it is in that phase.
Observing Venus through even the smallest of backyard telescopes can be rewarding. However, remember that at times the orbit of Venus will bring it near the sun so never slew (or point) your telescope at the sun without proper eye protection!
The clouds on Venus are fast-moving. Speeds of over 200 miles per hour are attained by the clouds there. Photographing clouds on Venus takes patience, a skilled observer and some experimentation with telescope filters.
The day-to-day changes on planet Venus are subtle, but in the end dramatic.
Former Arcadian Victor C. Rogus (F.R.A.S.) is a fellow of the Royal Astronomical Society, London, living in Sedona, Arizona.