Date: April 2, 2012
Title: Timekeeping on Mars
Podcasters: Nik Whitehead
Description: One day mankind will not only visit Mars, it will build permanent settlements on the Red Planet. When this happens the inhabitants of the new colonies will have to decide whether to keep the old Terran timekeeping system or to adopt a new system more suitable to their new home. Timekeeping on Earth is strongly based upon astronomically-defined intervals and so it is reasonable to assume that any Martian calendar will be based upon the astronomical phenomena visible from Mars. If this is the case, then what would a Martian calendar look like?
Bio: Nik is a lecturer in computer science at Swansea Metropolitan University in south Wales… but computer science is not her passion. She has a Bachelors degree in astronomy and astrophysics then took her Masters and Doctoral degrees in computer science when she realised that there are not enough jobs in astronomy to go around. What she’d really like to be when she grows up is either the navigator of the starship Enterprise or maybe a space traffic controller. In the meantime she’s working on visualisation tools for astronomical data.
Sponsor: This episode of the “365 days of Astronomy” is sponsored by — NO ONE! Please consider sponsoring a day or two in 2012 so we can continue to bring you daily “infotainment”.
Transcript:
Hello! Welcome to the 365 Days of Astronomy podcast for April 2nd. I’m Nik Whitehead, talking to you from South Wales in Great Britain, where I teach computer science at Swansea Metropolitan University. Although computing is my day job, astronomy is my first love, and I’m delighted to be able to contribute again to the 365 Days of Astronomy series.
When the Viking 1lander became the first object from Earth to land successfully on Mars on the 20th of July, 1976, NASA scientists faced an interesting problem: what system should they use to timestamp the masses of data that Viking and the missions that followed it would produce?
Recording the time of data collected on Earth is easy – it can be time-stamped using the years, months, hours, minutes and seconds of Universal Time. Universal Time is based upon the motion of the Earth in such a way that Earth’s sidereal day – the time taken for the Earth to rotate with respect to the stars – is 23 hours, 56 minutes and 4 seconds, while its solar day – from noon to noon – is 24 hours long. Mars takes longer to rotate on its axis so a Martian sidereal day is 24 hours, 37 minutes and 23 seconds, while its solar day is 24 hours, 39 minutes 35 seconds. The Martial solar day, or Sol, as it is known, is therefore approximately 2.7% longer than the Terran day.
NASA has defined Coordinated Mars Time as an equivalent to Universal Time. While Universal Time is based upon the point at which the sun crosses Earth’s reference meridian – at Greenwich in England – Coordinated Mars Time is based on a reference meridian that runs through the centre of the crater Airy-0. This hasn’t yet been used as the main method of timekeeping on any of the missions to the red planet, mainly because the position of the centre of the crater is not known to a high enough degree of accuracy.
Instead the NASA teams working with the Viking, Pathfinder, Mars Exploration Rover and Phoenix landers have used “Mars time”, wherein a Sol is made up of 24 Martian hours, each of 24 Martian minutes, but where the hours and minutes are each 2.7% longer than their Terran equivalents. A side-effect of the decision to use Mars time is that the landers’ operations teams have to arrive for their work shift 39 minutes later each day. This is necessary because the landers themselves can only work effectively during daylight hours when their surroundings are relatively warm. They also need sunlight to charge their solar panels, so they tend to work from local Martian dawn to Martian sunset. This dependence on local solar time meant that the Spirit and Opportunity rovers, on opposite sides of the planet, always worked during the other’s night-time.
No matter when the day starts, any Mars mission needs a method to keep track of the days as well as the hours. For now, each mission counts sols from the day on which they landed – although Viking and Phoenix defined touchdown day as sol 0, while Pathfinder, Spirit and Opportunity defined it as sol 1. All scientific measurements can thus be linked to a fixed point in time based upon the length of time beneath touchdown and the measurement being made. If scientists do need to refer to a time relative to the Martian year then they measure it in terms of the heliocentric longitude, or how far Mars has travelled around its orbit from the point of the northern Martian equinox.
Counting the sols from landing and using local solar time is fine at present, when our presence on the planet Mars is limited to a handful of active scientific missions. There will eventually come a time, though, when large numbers of humans will live on Mars, and they will have to come up with a system to coordinate time at a more human level. To this end, both scientists and science fiction writers have devised calendars for the red planet.
The simplest forms of these calendars attempt to maintain a similar clock cycle to the Earth by using Universal Time but then stopping at midnight for 38 minutes before starting again into the next day. While this has the advantage that noon on Mars is the same time as noon at Greenwich, it isn’t a very practical method if you want to do any science.
A more practical approach would be to use the same sort of Mars time that NASA uses for measuring time within a day – allowing for various time zones across Mars in the same manner that we have different timezones on Earth – but to define a new calendar system to count the days. Astronomers on Earth use the Julian calendar to record observations in terms of the number of days from a fixed starting point – astronomy’s touchdown day, as it were – but for day to day use we use the Gregorian calendar with its weeks and months.
The word ‘month’ has at its root the word ‘moon’, as the month was originally the period of time it took the moon to go from full to new and to full again. Although Mars has two satellites of its own, Phobos and Deimos, neither of these have orbital periods that make them suitable for timekeeping. Phobos’ orbital period is very short, so that is rises in west and sets in the east. Deimos has an orbital period that is close to a sol in length, so that Deimos rises, then remains in the sky for about two and a half sols.
Mars’ moons may not be any use for defining a mid-length period of time, but the Earth’s moon might be. The Earth and the Moon are easily visible from the surface of Mars, and the maximum angular distance between them varies between 3 and 17 arc minutes. This distance varies depending on how close Earth and Mars are to each other, but the 28-day cycle would be easy to observe. This has led people to suggest that a Martian calendar would have months similar to the Terran calendar.
An example of such a calendar is the Darian Calendar, devised by engineer Thomas Gangale in 1985. The Martian year is 668.59 sols, a number that splits neatly into 24 months. Each quarter of the year contains six months, five of which have 28 days and the final one has 27. The months of the Darian calendar are named after the signs of the zodiac, first in Latin and then in Sanskrit, so the month of Sagittarius is followed by the month of Dhanus – the Sanskrit name for that constellation – and then the month of Capricorn. Other similar calendars for Mars use the familiar western months but add a 1 or 2 to the end so that April1 is followed by April2 and then May1. Yet more simply number the months from 1 to 24.
Using a 28-day month is very convenient as it means that the 7-day week that we are all used to on Earth can be used on Mars. This is the approach that the Darian calendar takes, and it calls these days after the sun and the planets that are visible with the naked eye in the Martian sky – Mercury, Venus, Earth, Moon, Jupiter and Saturn. In months that have 27 days the final day is dropped so that the first day of the month is always on the same day of the week.
A different approach to developing a Martian timekeeping system was taken by Manfred Krutein. His system is heavily influenced by the decimal calendar introduced in France during the French Revolution. Instead of basing the Martian clock on the Terran one, with 24 hours, Krutein suggests that everything should be decimalised instead. A sol would be made up of 100 centisols starting at local noon. Each centisol would be 14 minutes and 12 seconds Earth time, and would be split into smaller amounts by using decimals. The sols of the year would be numbered from the northern hemisphere spring equinox (in the same way that the Gregorian year starts at the spring equinox in the northern hemisphere here on Earth.
So which calendar is likely to be used in practice? Well, given that the first humans on Mars will have been sent by a scientific agency such as NASA, the NASA version of Mars time is likely to survive for some time. In the longer term, when humans finally colonise the red planet I think that they’ll take their calendar with them and use a calendar that’s similar to the ones they’ve left behind on Earth, so something like the Darian calendar seems more likely than a decimal calendar.
Of course this is all academic until we do finally send a manned mission to Mars. Cuts in funding for such a mission in the West mean that this is unlikely to happen for at least another decade. Perhaps the months of the Martian calendar will be named in Chinese, not English. Whatever the language used, the really important thing is that any future Martian calendar is easy to use for both scientific and social purposes.
End of podcast:
365 Days of Astronomy
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