A Space Traveller's Guide to Mars, 1956

APPENDIX 1

It's About Time

by I. M. Levitt

Fels Planetarium
The Franklin Institute

A tremendous amount has been written concerning space travel. In 1950 a survey of many qualified scientists in the field was made, and their estimates as to when we will be able to get off the earth ranged from ten to 15 years to half a century. This question was posed to the author in 1955, and the time scale he devised has been accepted by many scientists.

If the unmanned, instrumented satellite is a reality in 1957 or 1958, he believed that by 1960 the television satellite will be placed in the sky to circle the earth at an altitude of 4000 miles in a four-hour orbit. This will telemeter back to the earth weather pictures of whole continents, thus making weather predictions a good bit more accurate.

By 1964 an instrumented satellite should go up and stay for some weeks or perhaps several months. This will be the "animal" satellite, for it will contain a full complement of mice, monkeys, and guinea pigs. These creatures will be carefully observed and their psychological and physiological reactions telemetered back to the earth. A comprehensive knowledge of the behavior of these animals will pave the way for the manned, instrumented satellite in 1968.

Again a satellite will get into the sky but this time with a human cargo— perhaps a single man or perhaps a small group. The important and significant knowledge of the human behavior to high accelerative forces and to the weird, unearthly feeling of gravity-free space will result from this adventure. There will be one major difference between this satellite and all the others which have gone up before. This one will have to return to the earth's surface. Building on this work will come the full-scale space station to circle the earth in a two-hour orbit at 1075 miles above the surface of the earth. The space station may be erected by 1978 or 1980.

Now will come a time of consolidations— time for restocking our scientific cupboard, and finally, by the year 2000, trips to the moon and planets are contemplated.

It can be assumed with considerable assurance that the first planet which will be visited and explored will be Mars. The choice is governed by our knowledge of the surface. It is certain that from what is already known of Mars today, it will be possible to land and take off from its surface.

One of the more necessary, significant instruments which the space traveler will need will be a timepiece— a clock. This will not be a simple type of clock but a complex and accurate one to indicate time and date, on both Mars and the earth.

"Why," the reader may ask, "will this be so important?" The answer lies in the tremendous speeds involved in space travel. There will come a time on Mars when the explorer will contemplate the return to the earth-circling space station. The instant the return has been decided is the instant he must know the precise time on the earth to determine the earth satellite position so that rendezvous with it can be effected. Depending on circumstances such as fuel supply, provisions, and other minor factors, missing the space station through faulty timing may doom an expedition. For this reason the precise earth time is secondary only to fuel and supplies to any group that leaves the earth.

How do we go about building this clock? The most important items which must be completely understood and incorporated into this clock are the rotation periods, the day of the earth, and Mars. Let's begin with the earth.

The earth spins on its axis once in 23 hours, 56 minutes, 4 seconds, which we call the sidereal day. This day is reckoned from the time a particular star appears on the meridian— that is, when it is due south— to the next time that star is on the meridian. However, the day we normally refer to, the day we live, is the mean solar day— that is, the day from noon to noon. This is the 24-hour day we use in our everyday lives.

As we have already discovered, Mars rotates on its axis once in 24 hours, 37 minutes, 22.6689 seconds. However, this is the sidereal day. The solar day is 24 hours, 39 minutes, 35.16 seconds long in mean solar units. Therefore Mars rotates slower than the earth; and if they were both to start off at the same time, then when the earth had made a complete rotation, Mars would still have slightly more than 39 minutes to go. Thus the day on Mars is about 4 per cent longer than that of the earth.

Imagine that you are on Mars and you have an earth clock. It is obvious that the clock will be a poor timekeeper for Mars. For instance, on the earth we are accustomed to eating lunch at noon. We might want to follow the same pattern on Mars. However, in using the earth time we would find that lunch the following day would be about 40 minutes earlier and lunch the day after about 80 minutes earlier. In fact, go on for about 18 days and you will be having lunch at the Martian midnight. This obviously won't do. Therefore it would be best to develop a clock which will keep time on Mars.

Arbitrarily we could divide the Martian solar day into 24 hours as in the case of the earth. Again, as in the case of the earth, we could have breakfast at 8:00 A.M., lunch at 1:00 P.M., and dinner at 7:00 P.M. As long as the Martian day was set at 24 hours, this procedure would work out fine. Now there is nothing to prevent our dividing the Martian day up into any intervals we wished, and when we divide the day into 24 hours, it follows the familiar pattern to which we are subject on earth.

Divide the Martian day into 24 hours and the Martian hours must necessarily be longer than those used on earth. If a clock is built, it must possess more than one dial to give simultaneously the time for both earth and Mars. The only problem would be a mechanical one in which the gearing for the two clocks would be in the ratio of the Martian day to the earth day. This, fortunately, is not too difficult a problem, but other complications present themselves.

Not only does the length of the day differ but the length of the month and the year are not the same. As we found out, because Mars is farther from the sun, it takes longer to swing around it, and therefore the year is longer— a year being defined as the time it takes a planet to completely circle the sun. The year is a fundamental period and also one of the most important timekeeping units.

Since Mars is about one and a half times as far from the sun as the earth, the year is almost twice as long. While the earth swings around the sun once in 365.242199 mean solar days (don't mind the big numbers; we are going to use all of them shortly), it takes Mars 686.979702 mean earth solar days. This corresponds to 669.599051 Martian sidereal days, or 668.599051 Martian solar days.

One interesting facet of the difference in the length of years is the age problem. Let's suppose you are 35 years old on the earth. On Mars you would only be about 19! And if you are 70 on earth, your Mars age would be almost 39. But before you begin to think the fountain of youth resides on Mars, it is well to contemplate that while the calendar indicates you are only 39, your body will insist you are 70. Incidentally the same thing is true of weight. If you are very much overweight, going to Mars will cut down your weight by over 60 per cent. But this won't do you a bit of good. You will still look precisely the same— so if you want to trim down, you will have to do it the hard way by actually dieting.

Now that the Martian year has been determined in rather precise units, let's build a Martian calendar.

A calendar must have a beginning, and arbitrarily this writer chose the date of the beginning of the Julian day epoch as the date of the beginning of the Martian calendar. This Julian day calendar should not be confused with the Julian calendar formulated by Julius Caesar in 45 B.C. The Julian day epoch was originated in honor of Julius Scaliger and was established in 1582. It simply expressed the date as the number of days which have elapsed since the beginning of the arbitrary "Julian era"— January 1, 4713 B.C. Thus that date also represents the year 0 and the beginning of the Martian calendar.

To begin the Martian calendar, an arbitrary date was chosen— the instant of midnight of December 31, 1953, the beginning of January 1, 1954; this was when the clock was designed. This corresponded to Julian day 2,434,743.5. The 0.5 must be used since the Julian calendar counts the day as from noon to noon while we count it from midnight to midnight. Now we are ready to construct our calendar.

Since:

2,434,743.5
668.599051

=

3641.5 plus, it means that January 1, 1954, falls in the Martian year 3641.

We adopt 668.6 for the number of Martian solar days in the Mars calendar year. The 0.6 days can be accounted for by permitting two years out of every five to contain 668 days, and the other three years will then contain 669 days. However, this calendar is too long! And it is too long by 0.6 - 0.59905 1, or 0.000949 Martian solar days per year. This means that the calendar will be out one day in about 1000 years. Therefore to keep the calendar in step we will drop one day every tenth century. Even with this adjustment there will still be a slight error. If from 0.001 (which corresponds to an error of one day in 1000 years) we subtract 0.000949, there results 0.000051 days. This means that the calendar will be wrong by this amount, but this amounts to only one day in 20,000 years. By comparison our Gregorian calendar, the one we use today, is out about one day in 3000 years.

There of course arises the question: Why do we have to have such a precise calendar for Mars? The reason lies in the fact that like the earth Mars has seasons. As we have read, the axis of Mars is tipped about 25 degrees to its path around the sun, and thus the sun will appear to climb up and down in the Martian sky during the Martian year. We have also discovered the Martian seasons are about twice as long as ours.

A calendar to be of any value must be synchronized with the seasons. To produce on Mars what corresponds to the earth's tropical year, it is necessary to include 0.6 day at the end of the year. This interval corresponds to the quarter days of our years which add up at the end of every four to give us an extra day in leap year. To take care of this 0.6 day the first and fourth year will contain 668 days while the second, third, and fifth will contain 669 days. In the longer year the extra day will be given to December.

The month used on the earth is the period of time during which the moon passes through its phases. The very word "month" is derived from "moonth," as it was called at one time. Because the moon passes through its phases in 29 1/2 days, that is the length of the terrestrial month. This is a most convenient period of time for activities, as it is an intermediate unit of time between the week and the year. In fact, centuries ago the month was an important interval, and events were depicted as occurring many moons ago.

Once we reach Mars, this almost natural unit of time loses its significance. We can of course try to use the moons of Mars. When we use the inner moon, the month is almost a half day in length; and when we use Deimos, the month becomes about five and a half days. However, these periods of time are too short to be considered of sufficient length for timekeeping purposes. Therefore the month on Mars will not be tied to a natural phenomenon.

This writer has given considerable thought to a calendar for the 668-plus days for Mars. It is possible to build a 12-inonth calendar for the planet. Let's divide up the Martian year into quarters of 167 days apiece. Then if the first and second months of each quarter are given 56 days and the remaining third month of the quarter is given 55 days, the four quarters can be given the necessary number of days to complete the year. In Martian leap years December will be given 56 days. Thus the 668-day year for Mars can be broken up into equal quarters and equal halves. And there will be a uniformity and regularity to the Martian calendar which we on earth will possess only when we adopt the World Calendar.

The week is an arbitrary unit of time on the earth. It was originally put into the calendar by the Babylonians, who saw seven objects in the sky which moved against the star background. These were the sun, the moon, and the five naked-eye planets. That is why we have a seven-day week. How fortunate we are that the ancient Babylonians did not possess telescopes. If they had and had discovered all the planets today, we might be using a ten-day week— which, by some, might be considered a little long, especially those who already consider the seven-day week too long.

We can have a seven-day week on Mars, too. This rather conveniently, for the months of 56 days will be divisible by seven exactly eight times. This works very well in the first and second months of the quarters. However, the third month has days, and here we run into trouble. Because the year are not divisible by seven, the Martian year, like the terrestrial one, will begin on a different day of the week every year.

A clock to demonstrate the earth-Mars time difference has been built by the Hamilton Watch Company under the guidance of the author. From the photograph it can be seen to possess a 15-inch face on which are found one primary and three secondary dials. The big dials graduated for a 24-hour day denotes the time on Mars. The secondary dial at 6 o'clock tells the earth time for the Greenwich meridian. The dial at 3 o'clock is really an earth-calendar dial indicating the month on the top slit, the year on the bottom slit, and the rotating hand moving against a dial and indicating the day of the month. This dial is calibrated to 31 days.

The dial at nine o'clock indicates the Mars date. The month is seen in the top slit, the year in the bottom slit, and again there is the rotating hand which gives the day of the month. This dial is graduated to 56 days. Automatic devices in the rear of the dial face automatically turn the month when the proper number of days have elapsed. When December has been run through, not only does the month change but the year counter turns over to add another year.

The names of the Martian months have arbitrarily been given the same names as those on the earth. The only difference will be a small "m" to indicate the Martian month so that January in the Martian calendar will be designated Janm.