How to tell time on Mars

This week, PhD Candidate Alex Innanen takes on a topic that has challenged planetary scientists and science fiction writers for decades: how do you tell time on a planet that has similarities in its revolutions to the Earth but some pesky differences? Often you'll hear of researchers working on Mars time because the length of its day is so similar to the Earth's. However, the match isn't perfect and can lead to unpleasant physiological effects for some and impractical, though hilarious, fixes. There is no 'Venus Time' or 'Jupiter Time' because their days are so different from ours that it makes no sense to try. But what to do with the extra 39 minutes in the martian day? Or the extra months needed in the martian year? 
(Image above from LMD's "Martian Seasons and Solar Longitude")

by Alex Innanen

One of my favourite things to read about are different calendars and methods of timekeeping. Here on Earth, there are all sorts of different calendars – the Gregorian, which you’re likely familiar with, the Julian calendar, and various lunar, solar or combination calendars. We also keep time within a single day in different ways – the standard now is a 24-hour clock, but at various times people have tried to introduce decimal time, where each day might have ten hours divided into 100 minutes. And this is just on our own planet, where generally the apparent movement of the sun and moon in the sky can give you a sense of when things are occurring.

Unsurprisingly, when you move to different planets things start to get more complicated.

Let’s take Mars for instance. Mars’ day (called a sol) is about 40 minutes longer than an earth day. If you want to use a 24 hour clock you either have to have a sneaky not-quite-an-hour-long 25th hour, or you have to make every hour a bit longer. Which is what the Mars clock that’s used for mission planning does. But wait, does that mean each hour has more than 60 minutes? Well, not really, if you make each minute a bit longer than an earth minute, and you can do that, not by adding extra seconds, but by making each ‘Mars second’ a bit longer than an Earth second. This way you can still use a familiar 24 hour clock.

This doesn’t mean that there’s some intrinsic ‘Martian second’ that is longer than an Earth second. While the second is an SI unit, it’s fairly arbitrary, as is splitting up a day into 24 hours. It’s just what we’re used to a second being.

One thing about round planets is that local noon – when the sun is directly overhead – happens at different times in different places. This is the reason that people on Earth invented time zones, so that local noon could line up – more or less – with noon on the clock. In reality, there’s a fair amount of variation even within a time zone. For example, there is about a 20 minute of difference between local noon in Toronto and Montreal just because of their difference in longitude, even though they use the same time zone.

There are no official time zones on Mars and missions tend to use local mean solar time (LMST), which is based on the average­ length of the sol, split into 24 hours. (There’s also local true solar time [LTST], which is referenced around the local noon, but drifts from 12:00 LMST throughout the year.) There is a generally accepted time standard for Mars, called Airy Mean Time (AMT) or Coordinated Mars Time (MTC) (comparable to Earth’s Greenwich Mean Time (GMT)/Coordinated Universal Time (UTC)) which refer to the mean solar time at Mars’ prime meridian, the crater Airy-1.

The other thing about Mars is its year is almost twice as long as an Earth year, about 668 sols or 687 Earth days. There is some familiarity, though, because Mars, like earth, has seasons! But Mars’ orbit is more eccentric than earth’s – that is to say, the path it travels around the sun is slightly more oblong. Earth also travels in an ellipse, and we do see the effects of not orbiting in a perfect circle on Earth seasons – the northern summer is about 94 days long, while the northern winter is only around 89 days – but not to so great an extent as on Mars. There the northern spring is 194 sols while the northern autumn is only 142 sols, a difference of 52 sols.

So what if we want to know what time of year it is? We can actually use the position of Mars (or any planet) in its orbit to tell this – specifically a parameter called solar longitude, shortened as Ls. We can start the year at a Ls of 0 degrees, the northern vernal equinox (the start of northern spring). Each subsequent season then starts at intervals of 90 degrees for a full 360 degree orbit. This convention is used in most scientific contexts because it can fairly easily tell you if events are occurring at around the same time year after year. It also avoids having to deal with things like leap years or leap seconds, because Mars’ orbit will always be 360 degrees – that’s just how geometry works.

All of this is what’s used currently to orient ourselves in time on Mars, but there have also been a number of attempts to create calendars and timekeeping systems that could be used by people living on Mars itself by everyone from scientists to fiction writers. The calendars divide the Martian year into months – which vary in their length, and in how many months there are in a year – and weeks – again, varying in length and quantity. Some calendars use familiar names for months and days of the week, and some make up new names, or new versions of the earth names. Getting into them all would probably take a whole series of blogposts, but it is a very fun rabbit hole to disappear down.