Reflections on the beauty of planetary motion

Why does the view of the moon attract our gaze and silence? What is there in the planets, these unusually bright points of light moving among the stars that we instinctively think is so beautiful? Perhaps they make us dream of distant, unattainable places? Perhaps the fact that they serve as lighthouses in the night, natural tongues of candles, allow us to ward off fear and give us hope in darkness? Or, perhaps, the fact that they seem to be floating in the air - and we like such things, be it autumn leaves, balls, clouds, or birds - suspended in the air, as if denying the force of gravity that holds us on Earth?

This last thought contains an interesting irony, because in fact the planets and the moon, traveling over our heads, obey the laws of gravity.

Periodically, it happens that our natural satellite, on its monthly journey around the Earth, passes alongside three bright planets in the sky, creating a beautiful spectacle. Naturally, the Moon does not physically go near these planets at all, just as your outstretched palm that covers the Moon does not go near the Moon. It’s all a matter of perspective - or geometry, the play of light and the elegant choreography of our solar system. But this perspective is not so easy to feel. Our eyes are unable to perceive the depth for objects that are so far away, so our brain forms a two-dimensional picture of the three dimensions of the Universe, and projects the Moon, planets, stars, which are extremely far from each other, onto a psychologically flat black screen of the night sky. It is necessary to strain the brain very strongly in order to see everything as it really is, and not as it seems. And on this fact is also worth thinking about.

Once in two weeks, the spectators of the night sky are waiting for the spectacle of the fragile Moon, which is similar to the tip of the nail - so thin as if it is ready to break from touch. This happens twice in four weeks, once immediately before sunrise, and then two days later, immediately after sunset. Why? Simple geometry and reflected light. [The indicated dates and phenomena are characteristic for observers located in the latitude of the continental US / approx. transl.]

The moon and the planets do not glow independently; they simply reflect the light of the sun. What we see, their brightness - all this is determined by how much sunlight they reflect in our direction. And this number is also a matter of simple geometry, a consequence of where the Moon and the planets are in relation to the Sun and the Earth. If you think about it and arm yourself with a small telescope, you can even, to your complete satisfaction, find out that the knowledge you have gained at school is true: the moon orbits around the Earth, and the planets around the Sun, two of which are closer to him than the earth, and the rest - on.

To make sure that the moon moves around the earth, and the telescope is not necessary; you just need to take a closer look at a few already known facts that you may not have compared with each other. We all know about the "phases" of the Moon - periods during the four weeks of the lunar cycle, when it is full, half-shaped, or appears as a sickle. But have you noticed that these phases follow a very clear and simple sequence? The crescent of the moon is always visible immediately after sunset, or before sunrise; on a night like this, the moon is visible for an hour or two. Half the moon is overhead, either at sunset or at sunrise, and half the night is visible. The full moon is visible all night, rises at sunset, sets at sunrise, and is above head at midnight.


The phases of the moon originate from the overlap of the half of the moon facing the sun (illuminated by light and marked in a lighter color) and half the moon facing the earth (the part that we can see east of the red lines). When the moon is approximately between the earth and the sun, you can see a very small part of the illuminated half of the moon, and the moon looks like a sickle; when the Earth is approximately between the Moon and the Sun, the highlighted and visible parts coincide, and the Moon is full. In fig. distances are not to scale.

And did you notice that the illuminated part of the moon is always on the side closest to the sun? If it is evening now and the moon is visible, the lighted part of the moon always looks to the west (towards the horizon where the moon sits), and if the moon is visible after midnight, then its lighted part always looks to the east (when the moon rises).

The visible size of the moon in the sky never really changes (except for the optical illusion, when it approaches the horizon, it looks bigger - if you take a photo, you will see that in fact it is not bigger). The upper and lower points of the crescent moon are separated from each other by the same distance as the top and bottom of the full moon. This can also be confirmed if you are lucky to see the beautiful phenomenon of the "ash light of the moon", which is easiest to see before the moon becomes too bright - it occurs within a few days after the sickle-shaped moon. Ash light is the light reflected from the illuminated part of the Earth, which reaches the unlit part of the Moon, and then - back to the dark parts of the Earth, where it can be enjoyed - the dim glow of the dark part of the Moon, which shows that the crescent moon is still a sphere , and the same size as the full moon.



Such phenomena may indicate that the distance from the Moon to the Earth never changes much; and that during the full moon the Earth is between the Moon and the Sun, and when the Moon looks like a sickle, it is between the Earth and the Sun. This means that the moon orbits around the earth at a nearly constant distance from the earth with a cycle of about four weeks. And the fact that a narrow strip of the illuminated part of the moon has the shape of a sickle confirms the sphericity of the moon.

In contrast to this system, the behavior of Venus and Mercury in the sky at first seems rather strange. They are never directly overhead; and generally, at midnight, none of them will be visible in the sky.

Have you seen mercury ? Unless you looked for it specifically, then probably not. To see Mercury, one must look at it either immediately after sunset, or shortly before the rising of the Sun - but not on the same day, or even on the same month. For example, February 22 [date shortly after the release of the original article / approx. trans.], if you look shortly after sunset, no longer than 30 minutes after it, with a clear horizon, without trees or buildings obstructing it, clouds or mountains to the west, you will find a small visible light at a distance of several visible moon sizes to the left of the crescent moon point alone. This is Mercury - and it really looks like mercury [Mercury - Mercury, mercury - mercury, mercurial - live, mobile / approx. trans.], because it is rather dimly visible at dusk, and very quickly comes.


Mercury and Venus, whose orbits are closer to the Sun than the orbit of the Earth, cannot appear at night overhead; they set shortly after the Sun, or rise shortly before it. The angle between them and the horizon at sunset or sunrise time is never greater than shown in the figure (not to scale). At the highest position, Venus will be much higher than Mercury, which suggests that the latter is closer to the Sun. The distances and angles are approximate, the sizes of the planets and the Sun are not to scale.

Why is Mercury so hard to see? Its orbit around the sun is much closer to the sun than the earth's orbit, so it never moves far from the sun in the sky, and must either set right after it or stand just before it. In its orbit, it (like Earth), from our point of view, moves in such a way that it comes out from behind the Sun (where we cannot see it in principle) to the side that is visible in the evening sky at sunset; then it passes between the Sun and the Earth (and because of the brightness of the Sun it cannot be seen), and then appears in the morning sky, but not far from the Sun, and then it again retreats behind it and the cycle repeats again.

Venus is easier to see for several reasons. First, it is larger than Mercury, its size is comparable to Earth. Secondly, if Mercury is grayish, Venus is bright white; its atmosphere (with a monstrous effect of greenhouse gases that maintain its temperature at a level higher than in the stove), always covered with white clouds, has a well-reflecting surface. Third, its orbit is farther from the Sun than Mercury’s orbit, and it has two consequences: Venus comes closer to Earth than Mercury, which makes it seem bigger and brighter, and rises higher in the sky than Mercury, which It means that sometimes it can be observed for several hours in a row after sunset or before sunrise (although for the same reasons as Mercury, not at the same time). Because of this, it can be observed without much effort. And, inevitably, every time this bright spot becomes visible, the police receive calls complaining about a UFO - a surprisingly bright unidentified flying object, also known as Venus.

The decisive argument in favor of the fact that Venus and Mercury move in orbits around the Sun, and not the Earth, is that they, like the Moon, have their own phases - albeit with key differences. Passing through phases, they change size, unlike the moon, and they are not the brightest in the full phase, like the moon, but in the wide sickle phase. The following happens: first, Venus appears in the evening sky, dim, but fully illuminated. Then, in the following weeks, it rises high in the evening sky, it becomes bigger, brighter, but not fully illuminated. Then she begins to go over the horizon in the evening, but becomes brighter and larger, approaching the shape of the sickle, and then, with a thin sickle, but of large diameter, it disappears in the twilight after sunset.



Then it appears a few days later at dusk in the form of a thin sickle, and the second half of the cycle occurs when it is seen early in the morning, in the sequence opposite to that of the evening festivities.


A planet with a smaller Earth orbit has phases similar to the Moon, but it grows and shrinks while moving in orbit due to a change in the distance to the Earth. The part of the planet that is visible from Earth is to the left of the dotted lines; the lighted portion is shown in yellow.

Unfortunately, it is impossible to confirm this with the naked eye — and this is probably what delayed the recognition of the fact that Venus has been orbiting the Sun for thousands of years. Only the telescope of Galileo, almost 400 years ago, discovered the phases of Venus described above.


From Galileo's notebook

A telescope is needed for this very small, such that it can magnify objects at least 20 times, or you can even do with binoculars (if you have good eyesight, the sky is clear and you can wait patiently for your eyes to get used to the darkness.


Photos of Venus

February 21 Venus will be in the middle of the evening part of the cycle. It will be lit more than half, its size will grow, the Sun will illuminate it less and less, and it will climb higher and higher in relation to the evening horizon - this spectacle is interesting in itself, and in the next few months it will become even more interesting. If you follow it from week to week, it will be seen how Venus will reach its highest point in relation to the horizon in about a month, then it will unfold and approach the Sun from our point of view. July 5-6 [2012] Venus will pass in front of the solar disk.

On February 25, the Moon will be located close to Venus. This week it will be the most interesting sight. When you see these two bright orbs in the sky, the spouse of Earth and her sister, think about what your brain does not want to tell you. Despite the apparent size of the moon and the fact that Venus seems to be a small point, this planet is actually five times larger than the moon (approximately equal to the Earth), and it seems smaller and dimmer only because it is now 400 times farther away.

Think about this as well. The light of the moon comes from the sun (and it takes him about 8.5 minutes, this distance is so great), reflects from the moon and comes to earth (it takes about 1.5 seconds), while the light of venus must pass from the sun (almost six minutes), reflect from Venus and go another eight minutes [according to the location of the planets in February 2012] to Earth. This is like an echo delay; when you hear thunder from a lightning strike in the mountains, you can first hear it directly, and then again, when the sound is reflected from a distant peak. If someone could turn off the Sun, you would see the Moon go out after eight minutes, and Venus will shine for another six minutes above that.



What about Jupiter? The largest planet of the solar system will continue to glow for another hour! This is exactly what is needed for sunlight to first reach Jupiter (it is about five times farther from the Sun than the Earth), and then reflected from it and reach the Earth (in February 2012 this distance is five times greater than from Earth to Venus) .

In late February, Jupiter will rise in the sky over Venus, and will be close to the zenith. February 26 The moon will pass next to them. Jupiter is much larger than the Earth or Venus - about 10 times larger in radius - therefore with a telescope or good binoculars (with a view that Jupiter is not a point, but a disk) it will look twice as large as Venus, despite what is 10 times further. And yet, despite the fact that he looks bigger, he seems dimmer than Venus. Why? The farther from the Sun, the more its light is scattered, so the Sun on Jupiter will appear dimmer and smaller than on Earth, and on Venus the Sun will look bigger and brighter. This means that less light is reflected from the surface of Jupiter. But do not let your mind fool you when on February 26 you see Jupiter small and dim compared to the Moon. Remember that inside this gas giant the Earth can fit a thousand times.

The fact that Jupiter is moving around the Sun, and that the distance to it from the Sun is much greater than to the Earth, can again be seen with a small telescope or even binoculars. Unlike Venus and the Moon, Jupiter has essentially no phases; it is always almost completely illuminated by the sun from our point of view. The same applies to Saturn (as well as Uranus and Neptune, which except through binoculars or a telescope and you will not see), and, to a lesser extent, Mars. All these planets, unlike Venus and Mercury, can be seen at midnight, and it is on these nights that they look bigger and brighter than anything. They are the smallest and dimmest when they are seen in the sky only immediately after sunset or shortly before sunrise.

Why it happens? The object is at its zenith at midnight when the Earth is located between it and the Sun, as in the case of a full moon. And this happens when these planets are closest to Earth. When they are visible only after sunset, they are very far from the Earth, and the Sun is almost between us and them, so they are rather dim, due to the huge distance between them and our planet. And since they never pass between the Earth and the Sun, they never look like sickles, as happens with the Moon, Mercury and Venus.


The change in the apparent size of Jupiter is shown in the top row of photos.



So if you find a telescope, you can enjoy the elegant dance of the planets and the moon; observe how the planets grow and shrink in the process of their approach and removal; note how they rise and set in our two-dimensional sky, and how this reflects their movement (and the movement of the Earth) in three-dimensional space; observe the phases of Venus and how they confirm the fact that it revolves around the sun. And you can ponder the fact that the small technical progress of 1609 strengthened Galileo’s vision just enough to allow him to discover so many previously unknown properties of the Moon, the Sun and planets, and forever change humanity’s view of himself and our heavenly neighbors.

But even without a telescope - with the naked eye or with the help of small binoculars - you can enjoy the beauty of the starry sky and plunge into the immensity of our house.

Watching the Moon and Venus, I will recall one of my favorite encounters with nature. It happened a few years ago, during a summer visit to the reserve surrounding Mount St. Helens , a volcano in southern Washington state that exploded in 1980. From the northern ridge, facing the gaping hole in the mountainside and the ruined valley below, I watched the stones turn grayish-golden at sunset, and waited for the stars to appear in the sky, which became almost black. In the west, Venus burned brightly, the pearl above and to the left above the bright pendants of the lunar crescent and the ash-gray moon. Then I heard a sniff, it seemed pretty close. I thought that this must be a small animal in the bushes behind me, but when I approached the bushes, I again heard this sound, already from another place. Surprised, I looked up, and was stunned. On the hill, no more than fifty meters away from me, against the background of the last orange glow of the sunset, the silhouettes of three majestic horned elks were visible.

They idly grazed while the Moon, Venus and the stars beckoning into the abyss hung above them, behind them.