All the planets will appear to line up across the sky towards the end of Februaryalxpin/Getty ImagesAll of our solar systems planets are lining up to parade through the night sky at once. This extraordinary celestial event will see the sky scattered with seven visible planets in what is known as a great planetary alignment.The eight planets in our solar system orbit the sun in roughly the same plane, because they all originally formed from the same disc of debris around the sun. The line the sun traces across the daytime sky, called the ecliptic, aligns with this plane, so when the planets appear in the sky, they all appear roughly along the ecliptic. It isnt a perfect line of planets, because their orbits are tilted slightly, but it is fairly close. AdvertisementNever is this more apparent than during a planetary alignment. An alignment including all of the planets except Mercury is taking place in mid-January. Uranus and Neptune, being the most distant planets, will only be visible through a telescope, but you may be able to spot the others with the naked eye.The great alignment, including Mercury, will only happen for a few evenings around 28 February, depending on your location. All seven planets will be visible briefly right after sunset, stretching in an arc across the sky.By the time the sky is completely dark, Mercury and Saturn will have sunk below the horizon, with Neptune and Venus following shortly after. The best time to spot the planets will be in the hour after sunset, when all of them except Mars, Jupiter and Uranus will be close to the horizon. Those three will continue to hang around for most of the night, but spotting three planets in the sky isnt nearly as rare as finding all seven. Voyage across the galaxy and beyond with our space newsletter every month.Sign up to newsletterThe main thing preventing such alignments from being visible all of the time aside from weather is the difference in orbital periods between the planets. Mercury, which is closest to the sun, takes about 88 Earth days to complete an orbit, while Neptune, which is most distant, takes nearly 165 Earth years.A great alignment is only possible when the planets are all relatively far from the sun, so they are visible at night, and all in roughly the same half of the sky, so they can be seen at the same time. It is a remarkable orbital coincidence sometimes there are multiple great alignments in a year, and sometimes several years pass without a single one.In some ways, a planetary alignment is simply an optical illusion: the planets are still separated by millions or billions of kilometres, and if you could look down on our solar system from outside of it, you would never see them arrayed in a perfect line emanating from the sun. But for stargazers around the world, its an excellent chance to see all of the planets at once, neatly arrayed across the sky.Topics: