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19 July 2018

Planets: the Great Stars of Summer

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Planets are best enjoyed in the summertime. Good weather, warm temperatures and some well-earned vacation time can help us to contemplate our fellow Sun orbiters with the naked eye; we need only dedicate a little time observing the spectacle of a starry sky that we had forgotten about over the winter. Summer of 2018 is even better than usual, because over the coming weeks we can enjoy an unusual sky with five planets visible at once: Mercury, 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 just like stars; however, they do things that stars cannot and that’s why they attracted our ancestors’ attention so much. Since the beginning of history, the “special powers” of planets have intrigued astronomers and prompted them to improve their theories about their strange behaviour.

Heading outside to observe planets is not particularly difficult since they look just like stars—you just have to know where to look. Venus and Jupiter are the easiest: when they’re in view, they’re the brightest objects in the sky; others like Mercury or Saturn are more elusive. But all of these (plus Mars) are within reach of the naked eye. It’s not necessary to use any special device, or even a telescope; just stay away from cities and population centres to avoid light pollution.

1. To find the planets you have to locate the imaginary line or route along which they travel, the ecliptic. All the planets are always close to this imaginary reference line that crosses the sky from northwest to northeast, at a medium altitude over the south during the summer nights. Thus the first step will be to look towards the south.

Planets show up (west to south) on the the path drawn by the zodiac constellations (Cancer, Leo, Virgo, Libra, …) | Sky over Madrid, 20 July 2018 at 22:00 | Source: Stellarium

2. Just in the twilight, looking west, where the Sun disappears, you will see a very bright object, in fact, the brightest object in the sky—the planet Venus. It is no coincidence that such a luminous object represents the goddess of beauty in various mythologies.

3. In the twilight, look down and to the right of Venus you may be able to see what looks like a weak star. This is Mercury, a very elusive and difficult object to see, so much so that it’s said that many astronomers have died without ever seeing it.

4. The easiest to locate is Jupiter, the point that shines above the south near the horizon. With the help of simple pair of binoculars, you can easily see four small objects surrounding it; these are 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.

5. Tougher to see is Saturn over the southeast, looking just like a second-rate star. It’s good to wait while the sky continues to darken a little more to appreciate its fainter shine.

6. To see the fifth planet we’ll have to wait a bit, but not too long; about one o’clock in the morning you’ll see an especially spectacular red star rising in the southeast. This is Mars and with this view you’ll have no doubt as to why it’s called the red planet.

Stargazing app using augmented reality | Credit:: Star Walk 2

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 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, 2018 and wait for the same day in 2019 and 2020 you will see that the positions of the stars and the constellations are identical. Same day, same time: 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 the five planets are visible at the same time, and also in summer, is a lucky coincidence in 2018 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 in exactly 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 that fixed background of stars. The closer the body is, the greater the variation: from night to night the change in position of the Moon is easily noticeable; the movement of the closest planets (Mars, Venus and Mercury) takes weeks to appreciate; and that of Jupiter and Saturn, months.

Mars orbit should be like this if it spun around Earth, in order to explain the planet’s irregular path in the sky | Credit: Johannes Kepler

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.)

Mars’ irregular path in the sky (above) is much better explained considering that both Earth and Mars spin around the Sun (below) | Crédito: B. Crowell

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.

Borja Tosar


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