ASTR-401 Lessons

Seeing Venus from Earth

Oops! I must have miscalculated slightly, because our spaceship isn’t as near to Venus as I’d hoped. As we make the final approach, though, it gives me just enough time to share some fast facts about the planet often called Earth’s sister.

Venus has always dazzled its observers. Named after the Roman goddess of love, prosperity, and victory, Venus has been an object of wonder in the night sky for centuries. Like Mercury, Venus follows the path of the Sun very closely and is only ever visible at dusk and dawn, with a maximum angular distance from the Sun of only 47.8 degrees. Ancient Greek astronomers believed that the morning and evening aspects of Venus were actually two different “stars” and named them accordingly: the morning Venus took the name Phosphoros or “dawn bringer,” and the evening Venus took the name Hesperos or “star of the evening.” It was the mathematician Pythias, also known as Pythagoras by the Muggles, who first recognized that they were both the same celestial body, so he is occasionally called the “discoverer” of Venus despite its previous incorporation into ancient astronomy.

Venus is the second planet from the Sun and the third brightest object in our sky, after the Sun and Moon. Its apparent magnitude ranges from -4.92 to -2.98. The full Moon is about 4000 times as bright as Venus at its brightest, which is, in turn, about 24 times as bright as the brightest star, Sirius. Like Mercury, Venus’s place between Earth and the Sun means that it can experience phases like the Moon when viewed from Earth. It takes about 584 days for Venus to go through its cycle of phases as seen from Earth. Interestingly, unlike Mercury, which is at its brightest when it’s in the full phase, Venus is at its brightest when it’s in a crescent phase. That’s because Venus gets closer to and farther from Earth than Mercury does. Its angular diameter, when seen from our planet, ranges from 9.7 arcseconds to 66.0 arcseconds (a far greater range than Mercury’s) so its distance from Earth more than compensates for its phase – unless, of course, too little of it is lit in the sky. How much of its surface is lit when it’s at its brightest? By observation, a little more than one quarter.

Venus looks bigger the less of it is lit in the sky.

Source: here

Venus is also notable for being the only planet named after a female deity. In fact, there aren’t many objects in the night sky named after women: Venus is joined by the constellations Andromeda, Cassiopeia, and Virgo, and the stars Bellatrix and Merope. Are the names of those two stars familiar to you?

Planetary Motions

Having touched on Venus’s revolution around the Sun, we now discuss its motion in more detail. Its orbit is closer to being circular than that of any other planet in the solar system. It has a year that is almost as long as an Earth year; it takes Venus almost 225 Earth days to make one full revolution around the Sun. A year on Venus is deceptive, however, because the year itself is shorter than a single sidereal day on Venus - that is, the time it takes for the planet to make a single rotation, which is a full 243 Earth days. It is also interesting to note that Venus rotates in the opposite direction from the rest of the planets. On Venus, the Sun would seem to rise in the west and set in the east – if it could be seen through the clouds! Another consequence of its rotation being in the opposite direction from its revolution around the Sun is that a solar day on Venus is shorter than its period of rotation: only about 117 days. 

Once a century Venus, as observed from Earth, passes across the face of the Sun; as you learned back in Year Two, Lesson Three, this movement of one celestial object in front of a larger one is called a transit. Venus’s most recent transit was in 2012. I went to a place where Muggles were gathering to observe it because some of them would bring telescopes that are better than mine - an unfortunate decision. The weather forecast was for mainly sunny skies, but the whole transit was blocked from view by a solitary cloud, so I had to content myself with watching it later on television. If I had stayed at Hogwarts, I could have flown above the clouds on my broom! I don’t expect to have another chance to see a transit of Venus directly because it will not happen again until 2117.

Venus’s position in the solar system also played an important role in understanding our own place in the solar system. When it transited the Sun in 1761 and 1769, astronomers all around the world looked up through their telescopes and noted the position of Venus and the time they made their observation. By comparing the positions of Venus relative to the Sun from different observation locations on Earth, the astronomers were able to determine the distance between Earth and the Sun – also known as the Astronomical Unit (AU).

Below you can see two images of the Venus transit in 2012. Can you notice where Venus is in the second picture taken by NASA’s spacecraft?

Transit of Venus

Source: here

Image of transit taken by NASA’s spacecraft

Source: here

What’s It Like on Venus?

Now that we’ve seen what Venus looks like from Earth and how it moves, we turn to the reasons why you wouldn’t want to go there. Venus is the planet in the solar system that is most like our own. It has a similar size – only slightly smaller than Earth – and even a similar composition. The similarities, however, are only skin deep. Venus regularly experiences temperatures up to about 460 degrees Celsius, or 860 degrees Fahrenheit – so hot that no life forms on Earth could survive it – and that’s at any time of day or night. The coldest place on the planet is the top of Venus’s highest mountain, Maxwell Montes (it is almost 11 kilometres, or 6.8 miles, high), where the temperature is “only” 385 degrees Celsius or 715 degrees Fahrenheit. This is all thanks to an extremely dense atmosphere made up almost entirely of carbon dioxide, which traps heat - causing what is known as the greenhouse effect. Its atmospheric pressure is about 92 times as great as that of Earth, while its surface is completely covered by clouds of sulphuric acid, which reflect about 76% of the sunlight that hits them and hide the surface from view. These highly reflective clouds are one of the reasons why, alongside its close proximity to our own planet, Venus appears so bright from here.

While Venus has no liquid water, it does have two continents – very high plateaus that tower above the rest of the planet’s surface. Venus also recently has had a lot of volcanic activity. This means that while other planets like Mercury or Mars have many impact craters, Venus can renew its surface, covering these craters under new lava deposits. As a result, Venus has a slightly smoother overall surface, though the many volcanoes mean that the planet isn’t completely smooth.

It is thought that Venus once had an atmosphere similar to that of Earth and may have even had liquid water, but that extreme volcanic activity created the carbon dioxide that stifled and scorched Venus. This activity implies that Venus used to experience plate tectonics, the movement of massive sections of crust on a molten layer of rock beneath, in the same way Earth does. Like Earth, the surface of Venus has undergone a great deal of change and very little remains of the surface from when the planet first formed.

Venus has a lot of surface features, including mountains, valleys and volcanoes. Interestingly, most of them are also named after mythical women - such as Ishtar Terra, a rocky highland area near the north pole of the planet. Its name comes from the goddess Ishtar - the Mesopotamian goddess of love and fertility. This feature is slightly larger than Australia; even bigger still is Aphrodite Terra, which is almost two times greater in diameter. It is named after the goddess Aphrodite - the Greek equivalent of the Roman goddess Venus. This feature is stretched across the equator and is covered with mountains and valleys. 

Another surface feature worth mentioning are Venus’s “pancake” domes, which are formed due to the extrusion of lava and can be found scattered on the surface of Venus, usually in groups. They have flat tops and steep sides, resembling shield volcanoes. 

The Carmenta Farra “pancake” domes

Source: here

Although some have proposed populating Venus through methods such as “floating cities” or “terraforming” (altering Venus’s climate and structure so as to resemble Earth’s), this is currently not a possibility outside of Muggle science fiction owing to Venus’s harsh environment. The discovery of phosphenes above Venus’s dense cloud layers has led to speculation that certain forms of life, particularly bacteria, might one day be found there, but up until now, we have discovered no conclusive evidence that any life could be supported on Venus.

Seeing Venus from Up Close

Not surprisingly, no manned missions to Venus have been planned, given the conditions there. Nevertheless, Venus is one of the most visited and viewed planets and has been the focus of several space probe missions. Mariner 2, launched by NASA in August 1962, became the first space probe to successfully visit another planet. The picture below shows what Venus looks like from above the clouds, as photographed by Mariner 10. It is a true colour picture, and it’s almost white and featureless, not orange as in some of the pictures you might have seen. Those pictures were taken by radar, which couldn’t have been in true colour.

Ah! Now we’re close enough to see Venus as seen by Mariner 10. This concludes the second leg of our tour. I’m sending the spaceship towards Mars’s orbit next, and this time we’re actually going to land on that planet.

Venus as seen by Mariner 10

Source: here

Mariner 10 was followed by many other space probes, such as the Soviet Venera program, which sent no fewer than 16 space probes to Venus between 1961 and 1984. Indeed, Venera 7 became the first spacecraft to land on another planet, and subsequent Venera missions returned valuable photographs and data that revolutionised our understanding of Venus while testing the engineering of the craft in extreme conditions. Venera 13 was the first spacecraft that succeeded in taking colour pictures of the planet.

The surface of Venus as seen by Venera 13

Source: here

A.M.E.

After discussing what could have been learned from a Muggle textbook, we turn to the most interesting subject for magical astronomers: Venus’s magical properties. As we learned in Year One, Venus’s magic influences the emotions, including love – the planet, after all, was named after the Roman goddess of love. In addition, it increases the pleasure you get from what you perceive through your senses, from tasty food and bubble baths to beautiful music, flowers, paintings, and people. Of course, it’s much stronger when you’re near Venus than when you’re seeing it from Earth (which may have been why I spotted some canoodling in the back of the class as we circled around the sunlit side of the planet).

So how strong is Venus’s magical effect on Earth? Unlike water clouds, through which magic can pass with almost no absorption, the sulphuric acid clouds surrounding Venus reflect magic as well as they reflect light, so the magical albedo of Venus as seen from above the clouds is the same as its optical albedo - about 0.76. This makes Venus’s magical effect on Earth the strongest of any of the planets, exceeded only by that of the Sun and the Moon. When is it the strongest? Venus is at its brightest when a little more than a quarter of its surface is lit. Its magical effect on the Earth is greatest when a little less of its surface is lit than that, because then the Sun appears closer to Venus in the sky, interfering constructively with Venus’s magic more strongly. The exact amount of Venus’s surface that needs to be lit for its magical effect to be strongest takes some complicated mathematics to calculate, but as long as about a quarter of its surface is lit in the sky, its magic is pretty close to the strongest it will get. In addition, if you have the newest version of the von Rheticus telescope, you can follow Venus’s A.M.E. Quotient from night to night and make your plans accordingly.

Nevertheless, as strong as Venus’s magic is at its greatest, it has limitations, as a certain seventh year Hogwarts student discovered to her chagrin. Sarah was fortunate enough to have one class – Charms – together with Raymond, easily the brightest student in his year. If she could get her hooks into him and encourage him to show enough ambition, she would have everything she wanted in life without having to be a superwoman, juggling a career and family. Despite her efforts, he hadn’t shown any romantic interest in her and she dared not resort to using Amortentia on him for fear of suffering Merope’s tragic fate, but one day she remembered an Astronomy lesson that she had attended three years previously, and it gave her an idea.

That evening, Sarah took her von Rheticus telescope, climbed the Astronomy Tower, and pointed it at Venus. About one quarter of it was lit in the sky, indicating that its A.M.E. was about as high as it ever gets. It would be above the horizon during the next Charms class, so its magic would surely reach Raymond. In preparation, she had her hair styled, and just before the Charms class, she dressed in her prettiest clothes, put on makeup and perfume, and cast the Concealing Charm on the only blemish on her face. At the end of the lesson, she sidled up to Raymond, smiled at him, and asked, “How do I look, Raymond?”

Sarah, all dolled up

Source: here

Sarah was so incensed that she could barely concentrate on the lesson enough to take notes. As soon as the lesson ended, she accosted Raymond in the corridor and exploded, “Last week you complimented me on my beauty, yet you choose to make out with a girl who not only has a plain-looking face and a boyish figure but makes no effort to improve her looks: no makeup and no style to her hair or clothes. What could you possibly see in her?”

Raymond smiled back and said, “You look especially beautiful today. What’s the occasion?” Venus’s magic seemed to be working.

Sarah didn’t come right out and ask him out on a date: her plan had a greater chance of success if he took the initiative. Instead, she hinted that she would be receptive to any invitation on his part by saying, “No special occasion, thanks. I just wanted to look good for you.” He didn’t ask her out, but she figured that he was too shy to do so in public. Perhaps he would send her an owl bearing the desired invitation, so she left her dorm window open for the whole week.

It never came. Just this once she was going to have to take the initiative. She arrived early for Charms class to get there before he did so that she could meet him in the corridor and ask him out privately. Eagerly she waited for his appearance. Venus’s A.M.E. would still be high and it would again be above the horizon. Surely its magic and her more direct approach would arouse his interest. But her hopes were shattered when she saw him walking along the hall holding hands with another girl and kissing her before heading into the classroom alone. Worse yet, that girl wasn’t even remotely pretty! How, Sarah wondered, could the planet’s magic have failed?

Sarah was so incensed that she could barely concentrate on the lesson enough to take notes. As soon as the lesson ended, she accosted Raymond in the corridor and exploded, “Last week you complimented me on my beauty, yet you choose to make out with a girl who not only has a plain-looking face and a boyish figure but makes no effort to improve her looks: no makeup and no style to her hair or clothes. What could you possibly see in her?”

Hortense

Source: here

“I appreciate your Veela-like beauty, but that’s not what I look for in a girlfriend,” he replied. “Hortense and I have a lot of interests in common, especially astronomy. She works so hard at it that she’s at the top of her class in that subject, like her role model, Dr. Ayesha Mansour. We have the same life goals too: a career as astronomers after we graduate from Hogwarts. It doesn’t pay all that well, but that’s of little importance to us. Neither of us has any desire to take a less interesting job just to get rich – or to stay home and be supported by a rich partner.” Apparently, other things are more influential than planetary magic. You will meet these three young people next year in the sequel to this story.

With that, we end today’s lesson. There will be a quiz but no essay. All but the first essay in Year Four Astronomy have you landing a planet or a natural satellite, but you certainly would not want to land on Venus.

Original lesson written by Professor Gagarina.

Some of this lesson written by Professor Plumb.