For the second time this year, the five brightest planets can be seen at the same time. You can catch them looking towards the western sky after sunset. The planets will form an ascending line from the horizon.
Mercury and Venus are low to the west, with the bright Jupiter shining just above. Higher up in the northwest sky is Saturn, and completing the set of five is the red planet Mars, on top.
On Friday, October 12, a beautiful crescent moon is found just to the right of Jupiter. Keep watching the planets night after night and you can follow the progression of the Moon. As the Moon travels the Earth each month, its apparent motion in the sky is much faster than the slower motion of the planets in their orbits around the Sun.
By Monday, October 15, the Moon will have moved higher into the sky to sit near Saturn, and a few days later, on October 18, the Moon will be associated with Mars.
That will also be a perfect night to see the planets, since Venus and Mercury will be sitting side by side. Of all five planets, Mercury is the weakest and, therefore, hardest to see, so having bright Venus as a sign of Mercury is always an advantage.
In about a week, Venus, which has been the bright evening star for most of this year, will move toward the brightness of the Sun and come out of the night sky.
Five planets, two groups.
The planets have been dancing happily in the night sky for the past few months.
In July, they also gathered in the evening sky, but on that occasion they stretched across the sky. Mercury and Venus could be found in the west, while Jupiter, Saturn and Mars rose in the east.
As Mercury and Venus are the inner planets, orbiting closer to the Sun than the Earth, we only see these two basses to the west after sunset, or low to the east before sunrise. They are the planets that follow or lead the Sun.
In contrast, the outer planets of Mars, Jupiter and Saturn can travel through the sky, which is exactly what they have been doing since July. The trio has moved from east to west, and now they join Mercury and Venus to present the spectacle of the five planets.
There are more in the store
It may seem common, since the five planets have come back together in just a few months. But it is only possible because Jupiter and Saturn are currently on the same side of the Sun and, therefore, close to each other, relatively speaking.
The five planets have joined twice this year and twice in 2016, but before that there was a decade when it simply was not possible. The two gaseous giants were too far apart.
When Jupiter and Saturn are paired in the sky, it is only a matter of time before the other planets fall into the correct configuration to bring them all together.
The next time this happens it will be in July 2020, but it will be harder to see compared to this week. The planets will spread across the sky instead of grouping in the west as they are now. So it's still special to detect the five planets that come together. There is great satisfaction in being able to mark the five planets in a single view.
To a challenge?
Not only are the five easy-to-see planets visible in the evening sky, but they are linked by Uranus and Neptune to complete the planetary assembly.
These two ice giants that orbit beyond Saturn are modern planets. They were not known in antiquity because their discovery needed the help of a telescope and an understanding of gravity to know how the Solar System works.
But although you can not see them with the naked eye, Uranus is low in the east at dusk and Neptune is higher, approximately halfway to Mars.
The practiced observers, who see the sky from a place in a dark country, have been able to see Uranus at a glance, knowing exactly where to look. Through the binoculars, Uranus appears as a faint star, but a good telescope will show its slightly bluish disk.
It is better to wait until later in the night, when Uranus has risen, to try to observe it. But now is an ideal time, as the planet approaches the opposition on October 24, when it will be at its best.
Neptune is about the same size as Uranus but much farther away, which makes it harder to see. Even with a modest telescope, it appears as a bluish star, while the proper observation conditions and a high-quality telescope are needed to reveal the Neptune disk.
Finally, and not to be left out, even the dwarf planet Pluto joins the crowd. It is too small and distant to be seen, but it is currently halfway between Saturn and Mars.
Even with a high-quality telescope, Pluto only appears as a weak object similar to a star, and it will be a challenge for most to find it in its current position among all the stars near the bright Milky Way.
If you are ready for the challenge, a free astronomy program such as Stellarium is ideal to help locate the planets. But it is so rewarding to enjoy the five bright planets, observed since antiquity, which are briefly united in the western sky.
Tanya Hill is an honorary member of the University of Melbourne and a senior curator (astronomy) at Museums Victoria, Australia.
This article has been published by The Conversation under a Creative Commons license. Read the original article.