The over a few days early last month, I caught Venus and the Moon in a dance!
One day, early last month, I looked up the western sky in the early evening, and saw the planet Venus, with the Moon below it. The next day, I looked up again, and saw that the Moon had gone past Venus, and was above it. And I then thought, “Ah!, they are in a dance!”
Venus is, after the Moon, the second brightest object in the sky and is presently the first visible object in the western evening sky. Venus, like three other planets (Mercury, Earth, and Mars) consists rocks, although about 80% of its surface is smooth. Indeed, Venus is so similar to the Earth in many respects that it sometimes called the Earth’s “sister” or “twin.” Venus and Earth are the second and third, respectively, closest planets to the Sun, which all planets go around or orbit continuously. In addition, the diameter of Venus at its equator is only about 326 Km less than that of the equatorial diameter of the Earth. In the same vein, the density of Venus is less than that of the Earth by on 271 Kg per cubic metre.
But Venus and Earth also have significant differences. I recall attending a seminar many years ago at the then Geology Department at the University of Wisconsin-Madison on the findings of an un-manned mission to Venus. After the stunning presentation, an old man in the audience said something along these lines: “You know, another strange characteristic of Venus is that the day is longer than the year.” The first thing that crossed my mind when I heard that was people in Ballanghar, my hometown, will call me a liar if I said such a thing!
Although it might sound strange for a day to be longer than a year, the reason for it is simple. A day, the time it takes Venus to rotate around its axis (imagine an orange spinning around a stick that pierces it through its center; that’s rotation), is longer than a year (the time it takes Venus to orbit around the Sun once) because Venus takes longer (243 earth days) to rotate around its axis, than it takes (225 Earth days) to orbit around the sun.
Venus is also interesting because the Sun rises in the West, and sets in the East! However, the Sun would not be visible to a person standing on the surface of Venus because its atmosphere consists of a blanket of carbon dioxide gas, and thick clouds of sulfuric acid (yes, that acid you put in your car battery). The blanket of carbon dioxide gas also traps heat, making Venus one of the hottest planets, hotter than even Mercury, the planet closest to the Sun.
The other partner in the dance I saw over a few days last month is the Earth’s only natural satellite and next-door neighbor, our Moon. The Moon is relatively small, with a diameter of only 3,476 Km at its equator, compared 12,756 Km for the Earth, which is also 81 times heavier. The Moon orbits the Earth at an average distance of 384,000 Km, and affects not only the tides on Earth, but also slightly increases the day length on Earth.
The Moon does not produce its own light, so it is visible to us on Earth only because it reflects light from the Sun. Nevertheless the Moon is, at Full Moon, the brightest object in the night sky because it is relatively very close to the us, and the fact that it receives a lot of light from the Sun.
In addition to its brightness, the regularity of the Moon’s orbit around the Earth has made it a cultural reference, and influenced human societies for thousands of years. Thus, the Lunar Calendar is used to determine many national and religious holidays and festivals such as Ramadan and Eid, as well as the Chinese New Year, and Rosh Hashanah (the Jewish New Year). The Moon has for a long time been associated with insanity, with the words lunatic and lunacy both being derived from Luna, the Latin name for the Moon. In addition, the Star and Crescent is part of the flags of many Islamic countries, dating back to the Ottoman Empire.
Fortunately for us the Moon and Venus dance will, Inshah Allah, be repeated from October 8 to 10, 2021. On Friday, October 8, the Moon will be visible from around 07:30 PM to the right and below Venus in the western sky. Remember that Venus will be most visible object in the sky so all you need to do is locate it and look slightly below it and to the right to find the moon.
On October 9, the Moon will be slightly above Venus, and to its right. The two objects would have changed positions, as you would expect in a dance, by the next day, October 10. The Moon would then be above and to the left of Venus, and this configuration will remain in the night sky until the next dance.
As I dug deeper into the matter, I found that what I thought was a Moon-Venus dance is a side show. The real and incredible dance is the one of the Earth and Venus. Because Venus is closer to the Sun than the Earth, and is orbiting around the Sun at a faster rate than the Earth, it “overtakes” the Earth every 584 days. As it overtakes the Earth, Venus changes from being an “Evening Star” visible in the west after sunset, to the “Morning Star” which is also the brightest object that is visible before sunrise.
The real dance of the Earth and Venus is best illustrated by the pentagram of Venus. If you draw a straight line between the Earth and Venus and place a red ball at the center of that line, it will trace a path as the two planets orbit around the sun. As shown in this animation (https://tinyurl.com/bdewhd4c), that path traces a lines which form a five-point, star-like shape at the center, hence its name, pentagram.
And there you have it! What I thought was a one-off dance of Venus and the Moon led to my realization, using Stellarium (https://stellarium.org/), that there are more. The next Venus-Moon dances start later this year on November 6, and December 5. Please don’t be late!