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Throughout the year, as described by their orbits around the Sun, the relative positions between the planets of the solar system are changing and this causes some planets to be in more or less favorable positions for observation from Earth. Sometimes some of them are close to the line of sight to the Sun and are not observable. This is what happens now with Mercury, Venus and Mars. The most favorable positions to observe the inner planets (that is, those that have orbits closer to the Sun than that of the Earth) are the so-called maximum elongation. In these positions, Mercury or Venus are located at the maximum apparent distance, east or west, of the king star.

For the outer planets, the most favorable positions are those called 'opposition'. We say that one of these planets is in opposition when it is located in the direction exactly opposite to that of the Sun with respect to the Earth (which is then located between the Sun and the planet). There the planet is at the minimum possible distance from our planet, is illuminated frontally and is observable all night. These oppositions happen periodically, following a period called 'synodic'. The average synodic periods, which have been known since ancient times, are 780 days for Mars, 399 days for Jupiter, and 378 for Saturn.

Jupiter was in opposition on June 10, and Saturn was on July 9. From those dates, as they move away from the Earth, the brightness of the two planets is decreasing day after day and also their elevations are decreasing over the horizon. According to their synodic periods, these planets will never be in opposition until the summer of 2020, but they are still looking splendidly. That is why it is convenient to take advantage of these last days of August and the first of September to look up to the sky and still enjoy the generous brightness of the two giant planets.

Jupiter super bright

Jupiter is now about 750 million kilometers from Earth, between the constellations of Ophiuchus and Scorpio, a little higher than the reddish star Antares. It can be seen in the southwest from twilight until 1h in the morning. To locate it there is no possible loss, because these days it is the brightest star in the night sky (after the Moon). With good binoculars or a small telescope it is possible to observe its four largest satellites: Io, Europa, Ganymede and Callisto, which were discovered by Galileo in 1610, as well as its characteristic cloud bands and even its large red spot.

Saturn on your left

To the left of Jupiter, following the line of the zodiac, we can observe Saturn, about ten times less bright than the first. These days the lord of the rings is visible from twilight until 3 o'clock in the morning. At first glance it looks like a bright star more, than those that abound in the constellation of Sagittarius, but we can distinguish it from those stars because they flash, while the planets shine more stable, without flickering. Saturn is now about 1400 million kilometers away, almost twice as far away as Jupiter, to observe its rings and surface details a telescope of at least 50 magnifications is required.

The planets, the Milky Way and the Moon

The novilunio took place on Friday, August 30, so the moonless nights of the weekend are especially favorable for the observation of the two giant planets. In a very dark place, far from light pollution, we can appreciate that they are located on one side and another of the Milky Way, thus offering a beautiful show. It is convenient to make the observation at about 11pm, when the road to Santiago will be almost perpendicular to the horizon line.

If we keep an eye on the continuous movements that take place in the celestial vault, we will see that the Moon will approach day after day towards the two planets. On the night of September 6, the Moon, already in very high, will be placed between both giant planets, on the same line of the ecliptic.

Rafael Bachiller is an astronomer and director of the National Astronomical Observatory (IGN)

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