Some questions have more than one correct answer because ‘correct’ is often a matter of perspective. Which planet has the longest day in our solar system? That is a great example of such a question because it has two correct answers, depending on how you define ‘day.’ And yes, there is more than one way to define such a seemingly simple notion, so let’s get started with some astronomy basics.
Stellar vs Solar Day
Generally, when we say ‘day,’ we mean the period over which our planet returns to the same point relative to the Sun — say, from noon to noon. This definition is based on Earth’s orbital rotation around our star and considers both our planet’s axial tilt and orbit length. For us, this time takes habitual 24 hours.
Stellar day, also called sidereal, considers only the time a planet takes to make one full revolution around its axis without considering its position relative to our star. This is a more accurate measure of time from an astronomic perspective because it only considers each planet’s rotation cycle and speed rather than its position relative to the Sun. But as it happens, once a planet makes a full rotation, it reaches the same position relative to more distant stars, hence the name.
The majority of planets in our solar system, ours included, have more or less the duration of solar and stellar days, with the latter a few minutes shorter. But there are always exceptions to the rule; in our system, there are two — Mercury and Venus. Both record holders as far as the longest day is concerned; the first one has the longest Sun day and the second — longest sidereal one.
But since we already explained that stellar days are more accurate from an astronomic perspective, let’s dig a little deeper — why is Venus the longest day? And how come Mercury lags behind?
Venus Vs Mercury: Who’s the Real Winner?
If we’re talking about the longest day in our solar system compared to Earth or rather our default definition of a day when we see the Sun at the same point up above. Based on this, Mercury wins because its full solar day lasts 176 ‘earth days.’
A solar day in Venus lasts 117 days in ‘our’ time, with a Venusian night taking even longer. From our habitual definition of nights and days, night in Venus would last 121 terrestrial days, which is very long, but still comparable to its solar day duration.
Besides, Venus’s period of axial rotation (stellar or sidereal day) is way longer — actually, it is the absolute longest in our solar system and takes 243 days. Space resource Orbital Today explains that such slow rotation speed is due to Venus’s retrograde orbit — that is, it spins in a different direction, and for hypothetical Venusian observers, the Sun rises in the West.
So, both planets have the longest days, no matter how you look at it. Mercury has the longest duration for someone standing on its surface (if that was even an option), while Venus takes the longest rotation cycles around its axis.
But what about other notable record holders in our solar system? Let’s discuss a few more curious cases below.
More Longest and Shortest Record Holders
Now that we know the longest record holders, which planet has the shortest day? Here, the answer is not at all ambiguous, and the ultimate title goes to Jupiter. Both solar and stellar days on this gas giant last under ten hours in ‘our time,’ with solar days traditionally longer. If back at home, our solar day is 24 hours — exactly 4 minutes longer than our period of axial rotation, on Jupiter, this difference is even less notable — just one minute, with a full solar cycle measuring 9 hours and 56 minutes.
But we don’t only measure time in days, so which planet has the longest year in our solar system? That is Neptune, taking 165 terrestrial years to complete one orbit around our common star. At this point, you may wonder if there is any difference between solar and stellar years, and you have every right to do so, considering what we’ve just learned about days.
Once again, years can be measured as periods necessary for a planet to complete a full orbit around the Sun, and that would give us a stellar, aka astronomical, year. But it’s also possible to consider summer and winter equinoxes as we do on Earth — that is when the Sun reaches its peak point from our perspective.
However, the difference between solar, aka tropical year, and stellar one is insignificant on solar system planets, even on Venus, which seems to make a point of being exceptional. And no matter how you look at it, Neptune’s year lasts longest because of its long orbit and huge distance from our star.
So, after all, not all questions have more than one answer.