The planets are best enjoyed in the summertime. Good weather, warm temperatures and a well-deserved holiday can help us to observe our solar orbiting companions with the naked eye. All we have to do is spend a little time observing the spectacle of a starry sky that we had forgotten about over the winter. This summer of 2022 will be even better than usual, because over the coming weeks we will be able to enjoy an unusual sky with four planets visible at the same time: Venus, Jupiter, Saturn and Mars.
Planets are not famous for their brightness or because they stand out in the sky in a special way—they basically look the same as stars; however, they do things that stars cannot do and that’s why they attracted so much attention from our ancestors. Since the dawn of history, the “special powers” of the planets have intrigued astronomers and led them to improve their theories about their strange behaviour.
Going outside to observe planets is not particularly difficult, you just have to know where to look. Venus and Jupiter are the easiest: when in sight they are the brightest objects in the sky; others, such as Mercury and Saturn, are more elusive. But these four (plus Mars) are within reach of the naked eye. It is not necessary to use any special device, not even a telescope. Just keep away from cities and population centres to avoid light pollution.
To find the planets one must locate the imaginary line or path along which they travel, the ecliptic. All the planets are always close to this imaginary reference that crosses the sky from east to west, passing at a medium altitude above the south during summer nights. So the first step is to orient yourself towards the south.
During the nights of this summer 2022, the first planet to appear in the sky will be Saturn. It begins to rise on the south-eastern horizon at about eleven o’clock at night, appearing as another star in the constellation of Capricorn, and as the night progresses it moves higher in the sky, gaining altitude and moving southwards. This is the movement of all the stars and planets in the sky. The stars on the opposite side of the sky move down and are hidden in the west. As some rise, others set.
The next planet to join the night is Jupiter, rising in the east at about one o’clock in the morning. It is easy to distinguish, as it is the brightest object in the sky at that time. With the aid of a simple pair of binoculars, four small objects can easily be seen around it, not stars, but its largest moons, Ganymede, Callisto, Io and Europa. It is not arrogant to say that at this moment you are following in Galileo’s footsteps. And don’t forget that his telescope was much weaker than the worst pair of binoculars you might have sitting at the bottom of a drawer. So don’t hesitate to grab them and see what you can observe.
- To see the third planet of the night, we’ll have to wait a little longer, but not too much longer. At about two o’clock in the morning, a particularly red and spectacular star will rise on the northeastern horizon. It doesn’t shine very brightly, but its red colour stands out in comparison to the other stars, making it easy to find between Taurus and Aries. But it’s not a star, it’s Mars. And once you’ve seen it, you’ll have no doubt as to why it’s called the red planet. Its red colour defined its role in mythology and astronomy, as it’s a colour that is rare in nature and unsettling: the colour of blood, of desert regions and of fire. The Romans assigned it the divinity of Mars, their god of war, most likely because of the relationship of its colour with blood spilled in battle.
The last of the planets we will be able to see on this summer’s nights is Venus, which will rise on the northeastern horizon shortly before sunrise, a little before six o’clock in the morning. It is very easy to spot, as only the Sun and Moon are brighter. It is no coincidence that the third brightest object occupies an important place in various mythologies. In the case of Greek mythology, the planet was assigned to Venus, the goddess of love, beauty and fertility.
At this early hour, just before sunrise, in the summer of 2022, we will be able to see four of the five planets visible to the naked eye: Saturn, Jupiter, Mars and Venus. It is a good time to remember that imaginary line, the ecliptic: with the planets in the sky it is easy to see it, joining those four bright points. When the planets are all in the same field of view it is often called an alignment, but this is not correct. The planets are always aligned, close to the ecliptic. The difference is that on these nights they are very close to each other and that line is very clear.
To help us out with these instructions, we can always use our mobile phones. In the app stores one can find applications such as Sky Map or Stellarium Mobile for Android and Star Walk 2 for iOS that allow us to identify and find any object in the sky with augmented reality. Simply pointing our phone at the sky will show a star chart that labels the objects in sight.
All the nights of the same date are almost identical. If you observe the sky on July 20, 2022, and wait for the same day in 2023 and 2024, you will see that the positions of the stars and the constellations are identical. Same day + same time = the same stars. However, from one year to the next, you won’t see the planets in the same positions. In some years you might see one planet, while in others you’ll observe several or none. That four planets are visible at the same time, and also in summer, is a lucky coincidence in 2022 and a great reason to look up at the night sky and get excited about the heavenly bodies and the science that explains the universe to us.
Stars are very far away, so much so that in our sky they form a backdrop that revolves with a cycle that repeats itself exactly in one year. With this in mind, throughout the year we can observe how the planets—much, much closer than the stars—are constantly changing their position against this fixed background of stars. The closer the body is, the greater the variation: from night to night the change of position of the Moon is easily perceptible; the movement of the closest planets (Mars, Venus and Mercury) takes weeks to notice; and that of Jupiter and Saturn, months.
The astronomers of antiquity noted those positions and saw that, after months and months, all the planets drew an irregular path on the fixed background of stars, like vagabonds in the sky. In fact, planet means “vagabond or wanderer” in ancient Greek. An object that defies the cycles and the regularity of the night sky without a doubt had to be important, and for that reason our ancestors soon related them to gods.
Planets even entered daily life where today they remain hidden in the days of the week. Tuesday (day of Tiw, the Germanic god of war) is in honour of Mars; Wednesday (day of Wodin, the supreme Germanic/Norse god) is associated with Mercury; Thursday (day of Thor, the Norse god of thunder and strength) is equated with Jupiter; Friday (day of Frigga, the Germanic goddess of married love) is associated with Venus; Saturday comes from Saturn; and Sunday and Monday are for the Sun and the Moon, which were also considered planets in the ancient idea of a geocentric universe in which all the stars revolve around the Earth. (Uranus and Neptune had not yet been discovered.)
Although we continue using the geocentric planetary week, it was the observations of the planets by Tycho Brahe in the sixteenth century that provided a major piece of evidence showing that the Earth is not the centre of the universe, that the Sun is the protagonist of the solar system and our world a mere secondary player in the great cosmic play, as Copernicus dared to suggest. The irregularities in the movements of Mars—recorded with a level of precision still surprising today, as they were made without a telescope—gave Kepler a crucial clue to formulate the laws that govern the movement of the planets, satellites, spacecraft and everything else in space, which in turn inspired Newton to come up with his law of universal gravitation. Thus the observation of the planets has been one of the great engines of astronomy throughout history.
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