?Which planet has the longest day
With a day/night alternation of 176 Earth days, Mercury has the longest solar day in the Solar System.
Question from Brunella Authier, Laval (Canada)
If we talk about the time it takes for a star to make a complete revolution on itself – i.e. its sidereal day –, and the only planets whose rotation we can calculate – those of the Solar System – then it is Venus . “It’s the one that rotates the slowest,” confirms Sean Raymond, researcher at the Bordeaux Astrophysics Laboratory. With a rotation speed of only 6.52 km/h, Venus takes 5,832 hours and 33 minutes – approximately 243 Earth days – to make a complete revolution.
Why is it so slow? Like all planets close to the Sun, Venus is subject to significant tidal effects which tend to slow down its rotation. This being said, the sidereal day of a planet only takes into account its rotation. Now, while it rotates on itself, the star also moves around the Sun...
ALAMY STOCK PHOTO/HEMIS – NASA/J. HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY/CARNEGIE INSTITUTION OF WASHINGTON – B. BOURGEOIS
With a day/night alternation of 176 Earth days, Mercury has the longest solar day in the Solar System.
?Sidereal or solar day
Astronomers have therefore established another definition of a day, with our star as a reference.
It is the solar day – the one we usually use – which corresponds to the day/night alternation: the exact time interval separating 2 passages of the Sun at the meridian. For the most part, the planets in our System rotate in the same direction as their orbit – counterclockwise on themselves, and likewise around the sun (or clockwise in both cases, depending on where they are observed from). For the Sun to illuminate the same region again, the rotation must be greater than 360°. A solar day is therefore slightly longer than a sidereal day – 24 hours and 23 hours and 56 minutes respectively on Earth, for example. Except that Venus is an exception: its direction of rotation is reversed in relation to its orbit, so that a solar day only lasts 117 days! In terms of day/night alternation – and not rotation – it is Mercury and its solar day of 176 Earth days which holds the record.
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