Spaceflight, July 1988, pp. 278-283

The Lost Calendars of Mars

Copyright © 1988 by Thomas Gangale

What profit hath a man of all his labour which he taketh under the Sun?
One generation passeth away, and another generation cometh: but the Earth abideth forever.
The Sun also riseth, and the Sun goeth down, and hasteth to his place where he arose.
The wind goeth toward the south, and turneth about unto the north; it whirleth about continually, and the wind returneth again according to his circuits.
All the rivers run into the sea; yet the sea is not full: unto the place from whence the rivers come, thither they return again.
All things are full of labour; man cannot utter it: the eye is not satisfied with seeing, nor the ear filled with hearing.
The thing that hath been, it is that which shall be; and that which is done is that which shall be done; and there is no new thing under the Sun.
Is there any thing whereof it may be said, See, this is new? It hath been already of old time, which was before us.

--Ecclesiastes

Introduction

In "Martian Standard Time" (JBIS, June 1986) I briefly sketched the history of the measurement of time on Earth in order to illustrate the interplay of various human needs that any chronometric system, either terrestrial or unearthly, must satisfy. From this base I then described my proposed system for Mars and its relationship to the terrestrial system. In this article, I would like to tell you what I have learned of the history of the measurement of time on Mars. Who would have dreamed that such a history existed for a planet whose habitation by humankind is at least a generation in the future? I certainly did not, and it was only while in the last stages of putting together the material for the book Martian Standard Time, a greatly expanded version of the original JBIS article, that I stumbled upon an obscure reference to a Martian calendarwhich pre-dated my own Darian calendar by at least 30 years. Having done so much work on my own Martian chronometric system, I was compelled to find out what it was that had come before me, so I embarked on a search back into the past for the lost calendars of Mars.

The Lovelock-Allaby Calendar

I happened to come across The Greening of Mars at about the time that I was submitting my original article to the British Interplanetary Society. In their 1984 book, James Lovelock and Michael Allaby concluded as I did that Martian time must be based on the solar cycles of Mars, both the diurnal and the annual. Their Martian clock, like mine, was basically a 24-hour Terran clock reduced in speed by 2.74913 per cent to coincide with the length of the Martian mean solar day. I disagree with them in that I believe they were unduly pessimistic concerning the ability of human biological circadian rhythms to adapt to this very slightly longer day. Also, I cannot foresee that there would ever be justification to use Martian clocks anywhere off Mars except as necessary to reference events on Mars itself.

Although they noted the correct length of the Martian solar day as 24 hours 39 minutes 35 seconds and intended that their Martian calendar be based on the Martian solar year, they made the mistake of having 687 days in their calendar when this is in fact the number of Terran solar days in a Martian solar year. It was as obvious to Lovelock and Allaby as it was to me that the cycles of Phobos and Deimos are entirely unsuitable as the bases for units of time. But, whereas I chose to adopt an artificial chronometric unit based on the cycle of the Moon, there are no months at all in the Lovelock-Allaby calendar:

We do not count months. On Earth these are based, clumsily, on the orbit of the Moon. Indeed, we have two tiny moons that look about the size Venus looks when seen from Earth. A division of the year into months would force us to choose one in preference to the other, and that would cause endless wrangling among the Phobos and Deimos factions that would spring up instantly. Even then it would not be easy. Phobos orbits Mars three times each day, and Deimos takes rather more than a day to make a single orbit. Martian months would be rather different from Terran months. Perhaps we could use both and try to devise a double-month system. I cannot begin to imagine what that would be like.

Recognising the sociological necessity of the seven-day week, Lovelock and Allaby adopted this unit of time that has no astronomical analogue either on Earth or Mars. Their calendar thus consists of days, weeks, and years. They retained the same names of the days of the week that are used on Earth, and the weeks themselves were numbered from the beginning to the end of the year. As their fictional character explains:

My date of birth, for the record, was 3.68.06. That is to say, I was born on the third day (which we call Tuesday, as I said) of the sixty-eighth week of the sixth year of the century. We omit the number of the century, but I was born in the year 106.

Although Lovelock and Allaby were not explicit on this point, the above passage implies that they intended each year and each week to begin on Sunday. If we assume a year of 687 days as Lovelock and Allaby did, we find that seven divides this number 98 times with a remainder of one day. This last day of the year would in effect be an intercalary day, for in their system the dates of the last two days of the year and the first day of the following year would be 7.98 (a Saturday), 1.99 (a Sunday), and 1.01 (another Sunday). But if we correct their error and consider years consisting of 668 and 669 days, we find that the 96th and final week would be only three or four days long, and of course Lovelock and Allaby did not confront the social desynchronisation that might result from this. The Egyptians, for instance, who had five intercalary days at the end of their solar calendar, did not try to resolve this anomalous period into something resembling a regular work week, but rather dealt with it by instituting a festival of five days in celebration of the rising of Sirius above the morning horizon and the rising level of the Nile. The Darian calendar avoids this issue by having a six-day week at regular intervals throughout the year. It is not so much that I am against having a big blowout at the end of the year; I just prefer to have as symmetric a calendar as possible.

Lovelock and Allaby chose to begin their Martian chronology with the establishment of the first human outpost on Mars. They were unclear as to whether they intended the anniversary of this event to be New Year's Day on their calendar. In any case, since the beginning of their chronology is defined by an event that has yet to come about, their calendar cannot be referenced to the Gregorian calendar.

The Aitken Calendar

Before either man or machine ventured into space, before the first transistor and the first digital computer, the astronomer Robert G. Aitken envisioned a human civilisation on Mars and foresaw the need for a calendar based on the diurnal and annual cycles of that world. Aitken beat Lovelock, Allaby, and me to the punch by over three decades, but by the 1980's, when the subject of human expeditions to Mars and the colonisation of that world at last began to be given wide and serious consideration, his Martian calendar had been forgotten. The Aitken calendar is truly a lost calendar of Mars. It would have been easy to leave it buried, and certainly safer for the Darian calendar not to call attention to a potential rival, but regardless of whatever calendar the Martians eventually choose to adopt, the Aitken calendar is part of their heritage, and it would have been less than honest of me to conceal what I unearthed. Of course, only time will tell if my own work is in turn forgotten.

Table 1. The Aitken Calendar's Two-Year Cycle.

Odd-Numbered Years

 
Spring
Summer
Autumn
Winter
Qtrs. First Day No. of Days First Day No. of Days First Day No. of Days First Day No. of Days
1 Sun. 42 Sat. 42 Fri. 42 Thu. 42
2 Sun. 42 Sat. 42 Fri. 42 Thu. 42
3 Sun. 42 Sat. 42 Fri. 42 Thu. 42
4 Sun. 41 Sat. 41 Fri. 41 Thu. 41

Even-Numbered Years

 
Spring
Summer
Autumn
Winter
Qtrs. First Day No. of Days First Day No. of Days First Day No. of Days First Day No. of Days
1 Wed. 42 Tue. 42 Mon. 42 Sun. 42
2 Wed. 42 Tue. 42 Mon. 42 Sun. 42
3 Wed. 42 Tue. 42 Mon. 42 Sun. 42
4 Wed. 41 Tue. 41-42 Mon. 41 Sun. 42

It was in a 38-year-old book about Mars that I chanced across a passing reference to a Martian calendar, which attributed the invention to the astronomer Robert S. Richardson. Eventually, I tracked down Richardson's 1954 book, Exploring Mars, only to find that he named Aitken as the inventor of the calendar. [Richardson's August 1947 article, "Calendar for Mars," also discusses the Aitken calendar. I have since found Aitken's 1936 paper, "Time Measures on Mars." --TG] He did give his opinion on the Martian clock, however, recommending that it be simply a slowed-down version of the 24-hour Terran clock.

Richardson led up to his discussion of the Aitken calendar by entertaining various methods of dealing with the fractional portion of the 668.599-day year. The first scheme had common years of 669 days and every fifth year consisted of 667 days. In the second method, common years would be 668 days and every fifth year would contain 671 days. Aitken's solution was to have the years run alternately 668 and 669 days and insert an extra day in every year whose number was divisible by ten. This is strikingly similar to the Darian calendar, except that Aitken had his odd-numbered years 668 days long and his even-numbered years 669 days long, and therefore every tenth year had 670 days. In contrast, all odd-numbered years have 669 (an odd number) days in the Darian calendar, and except for those years divisible by ten but not 1,000, which also have 669 days, all even-numbered years have 668 (an even number) days. Thus the Aitken calendar had years of three different lengths, while in the Darian calendar years come in only two varieties. Lacking the 1,000-year correction factor, the Aitken calendar is still accurate to a day over that period of time.

Aitken also dispensed with months as Lovelock and Allaby did, and like them he retained both the seven-day week and the terrestrial names of the days of the week. But while the Lovelock-Allaby calendar divided the year into units no larger than the week, Aitken devised two intermediate periods of time. He first divided the year into equal quarters of approximately 167 days, which he called 'seasons' and named Spring, Summer, Autumn and Winter. Each 'season' he quartered in turn so that each sixteenth of a year contained about 42 days, or six weeks. These sub-divisions of the 'seasons' Aitken called 'quarters,' although this term usually connotes a quarter of a year.

Indeed, while he chose not to call them months, in order to keep the terminology in this article consistent, one can think of them as such. Now, owing to the fact that the naturally-occurring seasons of Mars are quite asymmetric, there would be a very poor correlation between these and Aitken's symmetric 'seasons.' Assuming that Aitken's calendar began with the vernal equinox, he would have his first day of Summer 27 days before the summer solstice, his first day of Autumn 37 days before the autumnal equinox, and his first day of Winter 12 days before the winter solstice.

Table 2. Conversion of Darian Calendar Dates to Numerical Values.
  Month
Day Sag Dha Cap Mak Aqu Kum Pis Min Ari Mes Tau Ris
1 1 29 57 85 113 141 168 196 224 252 280 303
2 2 30 58 86 114 142 169 197 225 253 281 309
3 3 31 59 87 115 143 170 198 226 254 282 310
4 4 32 60 88 116 144 171 199 227 255 283 311
5 5 33 61 89 117 145 172 200 228 256 284 312
6 6 34 62 90 118 146 173 201 229 257 285 313
7 7 35 63 91 119 147 174 202 230 258 286 314
8 8 36 64 92 120 148 175 203 231 259 287 315
9 9 37 65 93 121 149 176 204 232 260 288 316
10 10 38 66 94 122 150 177 205 233 261 289 317
11 11 39 67 95 123 151 178 206 234 262 290 318
12 12 40 68 96 124 152 179 207 235 263 291 319
13 13 41 69 97 125 153 180 208 236 264 292 320
14 14 42 70 98 126 154 181 209 237 265 293 321
15 15 43 71 99 127 155 182 210 238 266 294 322
16 16 44 72 100 128 156 183 211 239 267 295 323
17 17 45 73 101 129 157 184 212 240 268 296 324
18 18 46 74 102 130 158 185 213 241 269 297 325
19 19 47 75 103 131 159 186 214 242 270 298 326
20 20 48 76 104 132 160 187 215 243 271 299 327
21 21 49 77 105 133 161 188 216 244 272 300 328
22 22 50 78 106 134 162 189 217 245 273 301 329
23 23 51 79 107 135 163 190 218 246 274 302 330
24 24 52 80 108 136 164 191 219 247 275 303 331
25 25 53 81 109 137 165 192 220 248 276 304 332
26 26 54 82 110 138 166 193 221 249 277 305 333
27 2755 83 111 139 167 194 222 250 278 306 334
28 28 56 84 112 140   195 223 251 279 307  
  Month
Day Gem Mit Can Kar Leo Sim Vir Kan Lib Tul Sco Vri
1 335 363 391 419 447 475 502 630 558 586 614 642
2 336 364 392 420 448 476 503 531 559 587 615 643
3 337 365 393 421 449 477 504 532 560 588 616 644
4 338 366 394 422 450 478 505 533 561 589 617 645
5 339 361 395 423 451 479 506 534 562 590 618 646
6 340 368 396 424 452 480 507 535 563 591 619 647
7 341 369 397 426 453 481 508 536 564 592 620 648
8 342 370 398 426 454 482 509 537 565 593 621 649
9 343 371 309 427 455 483 510 538 566 594 622 650
10 344 372 400 428 456 484 511 539 567 595 623 651
11 345 373 401 429 457 485 512 540 568 596 624 652
12 346 374 402 430 458 486 513 541 669 597 625 653
13 347 375 403 431 459 487 514 542 570 598 626 654
14 348 376 404 432 460 488 515 543 571 599 627 655
15 349 377 405 433 461 489 516 544 572 600 628 656
16 350 378 406 434 462 490 517 545 573 601 629 657
17 351 379 407 435 463 491 518 546 574 602 630 658
18 352 380 408 436 464 492 519 547 575 603 631 659
19 353 381 409 437 465 493 520 548 576 604 632 660
20 354 382 410 438 466 494 521 549 577 605 633 661
21 355 383 411 439 467 495 522 550 578 606 634 662
22 356 384 412 440 468 496 523 551 579 607 635 663
23 357 385 413 441 469 497 524 552 580 608 636 664
24 358 386 414 442 470 498 525 553 581 609 637 665
25 359 387 415 443 471 499 526 654 682 610 638 666
26 360 388 416 444 472 500 527 555 583 611 639 667
27 361 389 417 445 473 501 528 556 584 612 640 668
28 362 390 418 446 474   529 557 585 613 641 669

[The 12th, 18th, and 24th months ware later changed from Vrisha, Asleha, and Ali to Rishabha, Simha, and Vrishika, respectively. The above table has been updated to reflect this. --TG]

Rather than having a six-day week at the end of each quarter as needed and thereby enabling each quarter to invariably begin on the first day of the week as in the Darian calendar, Aitken allowed the days of the week to regress through his calendar over a two year period. His odd-numbered years began on Sunday, and since each 'season' was 167 days--one day short of being evenly divisible by seven, his Summer began on Saturday, Autumn on Friday, and Winter on Thursday. In even numbered years, Spring began on Wednesday, Summer on Tuesday, Autumn on Monday, and Winter on Sunday. Since even numbered years contained 669 days, this last 'season' was a day longer than normal; 168 being divisible by seven, the following Spring also began on Sunday to begin the two year cycle again. A nice feature of the Aitken calendar was that within each of the 'seasons,' all of the six-week periods began on the same day of the week. Aitken treated the extra day that he inserted every tenth year as an intercalary day--having no day of the week--so as not to upset his biennial cycle. The leap day occurred at the end of Summer--halfway through the year--so Aitken called it Mid-Year Day and declared it a Holiday.

Aitken did not specify a starting year for his calendar, and so like the Lovelock-Allaby calendar, his cannot be correlated with the Gregorian calendar.

The Levitt Calendar and The Levitt-Mentzer Clock

While searching for the details of what at the time I believed to be Richardson's calendar, I discovered evidence of yet another lost Martian calendar--and the first construction of a Martian clock--invented by his contemporary I. M. Levitt. I also subsequently found that in addition to describing the Aitken calendar, Richardson mentioned the Martian calendar devised by Levitt in a footnote of Exploring Mars. Again, as with the Aitken calendar, Levitt's inventions were far enough ahead of their time that they eventually faded into obscurity; yet they deserve their place in Martian history, and a treatise on Martian time would be incomplete without a discussion of his work.

Levitt published his idea of a Martian calendar in the May 1954 issue of Sky & Telescope. [He also described the clock and calendar in his 1956 book, A Space Traveller's Guide to Mars. --TG] He devised yet a different method of intercalation from those discussed by Richardson, the one adopted by Aitken, and that which is employed in the Darian calendar. He specified a five-year sequence in which the first and fourth years were 668 days long and the other three years contained 669 days. Additionally, however, Levitt allowed for the omission of a leap day every 1,000 years, as does the Darian calendar, and therefore claimed for his calendar an accuracy of a day in 20,000 years.

Since the cycles of Phobos and Deimos are quite useless in connection with a calendar, and the lunar cycle has nothing at all to do with Mars, it was quite reasonable for Aitken, Lovelock and Allaby to do away with months on their Martian calendars. From a social scientist's point of view, however, would it not be important to retain a chronometric concept that has been a part of most human cultures for centuries and even millennia?

I believe that a major advantage of the Darian calendar over these two competitors is that it demonstrates that months need not be abandoned on Mars, and that in fact months fit very neatly into the 668.5990-day Martian year. Levitt also saw that the month was a desirable unit of time to transplant to Mars; however, whereas I based my Martian months on the original model--the period of the Moon's orbit around the Earth and arrived at a total of 24 months of approximately 28 days each, Levitt proceeded along a different line of reasoning and instead divided the Martian year into 12 months as do most calendars on Earth. Levitt's months are thus half as many and twice as long as mine, averaging just slightly less than 56 days each.

Compared with the four Martian solar calendars discussed so far--the Aitken, the Levitt, the Lovelock-Allaby, and the Darian--Levitt's is the most conservative. He retained the same names of the 12 months of the year that date back to ancient Rome. Like the other four Martian calendars, the Levitt calendar inherited the seven-day week from the Gregorian calendar, and like the Aitken and Lovelock-Allaby calendars, he preserved the Anglicised names of the seven days of the week. In my view, the similarity in nomenclature of any Martian calendar to the Gregorian calendar is an invitation to great confusion, and I have purposely avoided this in the Darian calendar. I have exported to Mars the concepts embodied in the Gregorian calendar where possible, improved upon them where I saw the opportunity, but I have changed the names of the months and the days to give the Darian calendar a distinctive Martian flavour and eliminate any possible ambiguity.

For all that, the architecture of the Levitt calendar is nearly identical to that of the Darian calendar. His calendar, like mine and Aitken's, divides the year into equal quarters of 167 days, except in the case of the last quarter of a 669-day year, which is 168 days, But as opposed to Aitken, who employed unvarying seven-day weeks throughout his calendar so that the days of the week slid backward from quarter to quarter, Levitt contrived a six-day week to end each 167-day quarter. Thus, just as in the Darian calendar a six-day week ends with Dies Veneris and is followed by Dies Solis, Levitt's six-day week ends with Friday and is immediately followed by Sunday. As a result, every month on the Levitt calendar begins with a Sunday, just as on the Darian calendar each month begins on Dies Solis. In fact, ignoring the differences in the names of the months of the year and the days of the week, if one were to bisect each month of the Levitt calendar, the resulting structure would be indistinguishable from the Darian calendar.

Table 3. Comparison of Martian Solar Calendars.
Calendar Aitken Levitt Moore Lovelock-
Allaby
Darian
Year Published 1954 1954 1977 1984 1986
Number of Months 16 12 18 0 24
Days per Month 41-42 55-56 37-38 --- 27-28
Weeks per Month 5.96875 8 --- --- 4
Weeks per Year 95.5 96 --- 96 96
Numerical Day of Vernal Equinox 1 545 --- --- 1
Roman Calendar Reference --- 1 Jan 4713 BC --- --- 26 Dec 1975 AD
Intercalation Sequence 668
669
668
669
668
669
668
669
668
670
668
669
669
668
669
668
669
669
668
669
--- --- 669
668
669
668
669
668
669
668
669
669
Accuracy 1,000 years 20,000 years --- --- 20,000 years

I chose to begin the Darian calendar with an historic Martian event and a Martian astronomical event: the vernal equinox prior to the landing of Viking 1 on Mars. Lovelock and Allaby based their Martian chronology on a Martian event that has yet to occur, and Aitken did not designate a beginning year for his calendar. Levitt, on the other hand, tied his Martian calendar to the beginning of the Julian Period--a terrestrial event--since no historical event had at that time taken place on Mars.

So while the Aitken and the Lovelock-Allaby calendars cannot be referenced to the Gregorian calendar, the Levitt calendar, like the Darian calendar, can be. Also like the Darian calendar, Levitt began his with the year 0 rather than 1. Thus January 1, 4713 BC on the Julian calendar was also January 1, 0 MY (Martian Year) on the Levitt calendar. Although Levitt did not furnish an exact correlation between his calendar and a modern date on the Gregorian calendar in his 1954 Sky & Telescope article, he did state that January 1, 1954 AD occurred in the year 3641 MY.

One of the most important aspects of a calendar is its relationship to the seasons, yet Levitt failed to mention whether his New Year's Day is in the Martian spring, summer, autumn or winter. It is possible to calculate the dates on which the equinoxes and solstices fall on the Levitt calendar and thus make it a bit more complete. Also, by so doing, we can establish a correlation between the Levitt calendar and the Darian calendar. Sagittarius 1, 0 ME on the Darian calendar corresponds to JD 2,442,771.657. Dividing this number by 1.0274913 yields 2,377,413.4, the number of Martian solar days that have elapsed since January 1, 0 MY on the Levitt calendar. If we divide further by 668.599, the integer portion of the quotient--3555--is the corresponding year on the Levitt calendar, and the remainder plus one--545--is the numerical day of the year. Since the structure of the Levitt calendar is so similar to that of the Darian calendar, Table 2 can be used to convert this numerical value to the corresponding Levitt calendar date if for the Sanskrit months we add 28, and for Sagittarius and Dhanasu we read instead January, February for Capricornus and Makara, et cetera. Thus Day 545 on the Levitt calendar, the date of the vernal equinox in the northern hemisphere, is October 44, and so December 26,1975 AD, Sagittarius 1, 0 ME, and October 44, 3555 MY are all equivalent dates. Further calculations of this nature show that January 1 on the Levitt calendar occurs in the late spring, with the summer solstice taking place on February 14. Similarly, May 24 marks the autumnal equinox and July 27 is the date of the winter solstice.


Note: Since the publication of this article, I have corrected this reference date/time for the Darian calendar to 1975 December 18 23:27:37, JD 2442765.47752. Additionally, I have since adopted the Telescopic epoch, making the year of the Viking landings 195. Finally, I have recalculated the nominal dates of the equinoxes, solstices, and apsides on the Levitt calendar as follows:

Aphelion January 28
Summer Solstice February 14
Autumnal Equinox   May 26
Perihelion July 28
Winter Solstice August 2
Vernal Equinox October 45


Now there is a serious discrepancy between the correlation of Levitt and Gregorian dates that Levitt himself reported and the result that I have reached here. How could January 1, 1954 AD be in the year 3641 MY if December 26, 1975 AD corresponds to October 44, 3555 MY? Levitt obtained his result by taking the Julian Day of January 1, 1954 AD and dividing this figure by the number of Martian solar Days in a Martian solar year. Levitt's error was in not also dividing by the ratio of Terran solar days to Martian solar days, or alternatively he could have divided the Julian Day by the number of Terran solar days in Martian solar year to obtain the correct Martian Year of his calendar. It turns out that January 1, 1954 AD on the Gregorian calendar actually corresponded to December 39, 3544 MY on the Levitt calendar.

Hopefully, this error appeared only in Levitt's Sky & Telescope article and was not incorporated into the design of the Earth-Mars clock. Designed by Levitt and constructed in the home workshop of Ralph B. Mentzer of the Hamilton Watch Company, the clock was unveiled at the Waldorf-Astoria Hotel in New York on February 14, 1954. The main dial of the clock displayed 24-hour time on Mars; whether or not this was intended to be Martian Prime Meridian Time is unclear. Additionally, the clock had three smaller dials on its face: the first marked the day, month, and year on Mars according to the Levitt calendar; the second displayed 24-hour GMT; the third showed the day, month, and year according to the Gregorian calendar. The Levitt-Mentzer clock was capable of being run forward or backward at 2,000 times normal speed and stopped at any date between January 1, 1970 and December 31, 1989, and thus functioned as a mechanical analog computer for relating time on both worlds. Recall that at the time Levitt and Mentzer devised their clock, electronic digital computers were in their infancy.

And it was not only electronic computers that were in their infancy in 1954 when Richardson discussed Aitken's calendar in Exploring Mars and Levitt published his own calendar in the May issue of Sky & Telescope. I have always been one of those who dreamt of going to Mars and, strangely enough, I have always been very interested in calendars as well. Perhaps there was something in the air at the time of my birth. You see, on the Darian calendar I was born on Vrisha 4, -12 ME [since recalculated as 183 Gemini 11, Telescopic epoch], which on the Levitt calendar would be February 30, 3545 MY; but of course most Terrans are more familiar with their own Gregorian calendar, according to which my date of birth is May 13,1954 AD.

The Moore Calendar

Having now found three Martian solar calendars, I resolved to make a general search; how many more were out there, I wondered?

The Viking landings on Mars inspired an invasion of the bookstands by a host of books about Mars. One of them was Patrick Moore's 1977 Guide To Mars, and in it was still another Martian calendar. Moore's idea was to divide the Martian year into 18 months, all but three of which would be 37 days long. The 6th, 12th and 18th months were instead 38 days long. Moore made no mention of weeks in his calendar, and indeed, since 37 is a prime number, there is no hope of having an integral number of weeks, seven-day weeks or otherwise, in his months. Yet the sociological need for this unit of time is incontrovertible, and certainly the Martians will not do without it. Also, Moore did not take on the problem of intercalation to account for the fractional number of days in the year.

Conclusion

Although I had read many books and magazine articles about Mars before I began my work on Martian time, I was somehow unlucky enough never to have come across even one of these other Martian calendars. Or perhaps this was good fortune, for had I known in advance of a Martian chronometric system that pre-dated mine, I might have been daunted from plunging into the task with such zest as I have done in my happy ignorance. What I had assumed was a brand new field I have since come to see is already a rather crowded one, and there may be still other Martian calendars out there somewhere. But of all the ones that I have found, none can claim all of the features that are incorporated into my own Darian calendar:

  1. A seven-day week.

  2. An integral number of weeks per month, enabling each month to begin on the first day of the week.

  3. A 28-day month that approximates the familiar lunar cycle.

  4. A year that begins on the vernal equinox, symbolic of the beginning of life.

  5. The simplest possible intercalation sequence, requiring calendar years of only two different lengths.

  6. An accuracy on the order of one day in 20,000 Martian years.

  7. A date and time reference to the Roman calendar based on a Martian historical event, enabling any occurrence to be expressed in either system.

  8. A computer program that prints calendars and generates monitor displays correlating Martian and Terran dates and times.

  9. A distinctive nomenclature that precludes any possibility of confusion between the Martian and Terran chronometric systems.

And with these proud strokes of the word processor keys I thought I was finished writing this article. I had looked through every astronomy book about Mars I could find, but could uncover no more calendars. The next morning, in the last REM period before waking, I dreamt of a calendar in Robert A. Heinlein's 1949 Red Planet, a juvenile novel I had not read in 20 years. I bounded out of bed in the predawn gloom, flipped on the light in our study, and instantly found the book, for I had earmarked it for re-reading as part of the research for a future writing project of mine. Sure enough, in the course of a conversation in Chapter 1:

Jim thought back over the twenty-four months of the Martian year. 'Since along toward the end of Zeus, nearly November.'

'And now here it is the last of March, almost Ceres, and the summer gone.'

Except for references to days of the week, named the same as the ones we know on Earth, Heinlein made no further mention of a Martian calendar, but the above passage proves that my invention of a 24 month Martian year, like many of the features in the Darian calendar, is not a new one under the Martian Sun.

Bibliography

Heinlein, Robert A., Red Planet, Charles Scribners Sons, New York, 1949.

Levitt, I. M., "Mars Clock and Calendar," Sky & Telescope, May 1954.

Lovelock, J., and Allaby, M., The Greening of Mars, Warner Books, Inc. New York, 1984.

Moore, Patrick, Guide to Mars, W. W. Norton & Company, New York, 1977.

Richardson, Robert S., Exploring Mars, McGraw-Hill Book Company, New York, 1954.

New York Times, "Mars Clock in Debut," February 15, 1954.