(Picture above from LMD’s “Martian Seasons and Photo voltaic Longitude”)
by Alex Innanen
Certainly one of my favorite issues to examine are totally different calendars and strategies of timekeeping. Right here on Earth, there are all kinds of various calendars – the Gregorian, which you’re seemingly aware of, the Julian calendar, and numerous lunar, photo voltaic or mixture calendars. We additionally preserve time inside a single day in several methods – the usual now’s a 24-hour clock, however at numerous occasions folks have tried to introduce decimal time, the place every day may need ten hours divided into 100 minutes. And that is simply on our personal planet, the place usually the obvious motion of the solar and moon within the sky may give you a way of when issues are occurring.
Unsurprisingly, whenever you transfer to totally different planets issues begin to get extra sophisticated.
Let’s take Mars as an example. Mars’ day (known as a sol) is about 40 minutes longer than an earth day. If you wish to use a 24 hour clock you both need to have a sneaky not-quite-an-hour-long twenty fifth hour, or it’s important to make each hour a bit longer. Which is what the Mars clock that’s used for mission planning does. However wait, does that imply every hour has greater than 60 minutes? Properly, probably not, in the event you make every minute a bit longer than an earth minute, and you are able to do that, not by including additional seconds, however by making every ‘Mars second’ a bit longer than an Earth second. This manner you may nonetheless use a well-known 24 hour clock.
This doesn’t imply that there’s some intrinsic ‘Martian second’ that’s longer than an Earth second. Whereas the second is an SI unit, it’s pretty arbitrary, as is splitting up a day into 24 hours. It’s simply what we’re used to a second being.
One factor about spherical planets is that native midday – when the solar is immediately overhead – occurs at totally different occasions elsewhere. That is the explanation that folks on Earth invented time zones, in order that native midday may line up – kind of – with midday on the clock. In actuality, there’s a good quantity of variation even inside a time zone. For instance, there’s a couple of 20 minute of distinction between native midday in Toronto and Montreal simply due to their distinction in longitude, regardless that they use the identical time zone.
There aren’t any official time zones on Mars and missions have a tendency to make use of native imply photo voltaic time (LMST), which relies on the typical size of the sol, cut up into 24 hours. (There’s additionally native true photo voltaic time [LTST], which is referenced across the native midday, however drifts from 12:00 LMST all year long.) There’s a usually accepted time normal for Mars, known as Ethereal Imply Time (AMT) or Coordinated Mars Time (MTC) (corresponding to Earth’s Greenwich Imply Time (GMT)/Coordinated Common Time (UTC)) which seek advice from the imply photo voltaic time at Mars’ prime meridian, the crater Ethereal-1.
The opposite factor about Mars is its 12 months is sort of twice so long as an Earth 12 months, about 668 sols or 687 Earth days. There’s some familiarity, although, as a result of Mars, like earth, has seasons! However Mars’ orbit is extra eccentric than earth’s – that’s to say, the trail it travels across the solar is barely extra rectangular. Earth additionally travels in an ellipse, and we do see the consequences of not orbiting in an ideal circle on Earth seasons – the northern summer season is about 94 days lengthy, whereas the northern winter is just round 89 days – however to not so nice an extent as on Mars. There the northern spring is 194 sols whereas the northern autumn is just 142 sols, a distinction of 52 sols.
So what if we need to know what time of 12 months it’s? We will truly use the place of Mars (or any planet) in its orbit to inform this – particularly a parameter known as photo voltaic longitude, shortened as Ls. We will begin the 12 months at a Ls of 0 levels, the northern vernal equinox (the beginning of northern spring). Every subsequent season then begins at intervals of 90 levels for a full 360 diploma orbit. This conference is utilized in most scientific contexts as a result of it could possibly pretty simply let you know if occasions are occurring at across the identical time 12 months after 12 months. It additionally avoids having to cope with issues like leap years or leap seconds, as a result of Mars’ orbit will at all times be 360 levels – that’s simply how geometry works.
All of that is what’s used at the moment to orient ourselves in time on Mars, however there have additionally been plenty of makes an attempt to create calendars and timekeeping techniques that may very well be utilized by folks residing on Mars itself by everybody from scientists to fiction writers. The calendars divide the Martian 12 months into months – which range of their size, and in what number of months there are in a 12 months – and weeks – once more, various in size and amount. Some calendars use acquainted names for months and days of the week, and a few make up new names, or new variations of the earth names. Moving into all of them would most likely take a complete sequence of blogposts, however it’s a very enjoyable rabbit gap to vanish down.