原文: Mars Time or Earth Time?

What day is it? Do days exist without calendars? Does time pass when there are no human hands left to wind the clocks?

- Howard Koch, Invasion from Mars, the 1938 radio play based on H. G. Wells' The War of the Worlds

It didn’t get a mention on Entertainment Tonight, but, October 29th was a special day for the gang over at JPL as the Mars rover Opportunity turned two.  Too late now to send a card, but imagine my confusion: the launch date was July 7th, 2003 with orbital insertion on January 25th, 2004.  Ignoring the fact that planetary survey vehicles might count birthdays more like dogs or woman over thirty, the real problems start when keeping track of time on Mars instead of good old Terra.  It turns out, the occasion celebrated by the spry robot was the beginning of its’ third Martian year roughly equal to 687 earth days.

The first problem is what constitutes a day since one can measure either sidereal days or solar days. On Mars they differ by a little more than two minutes but are both just over half an hour longer than on earth.  At least the basic units on our digital watches are the same.  Scientists finally agreed to the shorthand of calling each box in their red planet day planner a ‘sol,’ which is certainly easier to use in equations than 88,775.24409 seconds.

The NASA Mars operations team coined the word “yestersol” to refer to the previous sol which is a lot more fun to say than the slew of acronyms normally tossed around by space geeks.  Apparently, the press even picked up on the new lingo, though I can’t recall ever hearing it on the evening news.  The related neologisms “tosol” (for “today”) and “morrowsol” or “nextersol” (for “tomorrow”) never seemed to catch on, that is until ‘nextersol’ when this post is uploaded.

The number of ‘sols’ a spacecraft lander spends making tracks in the crimson dust must next face the same issue in the 2000 vs 2001 millennium debate: do you start counting from ‘0’ or ‘1’ ?   The two Viking missions commenced from ‘Sol 0’ after touchdown, while Pathfinder and the two MERs defined the same moment as ‘Sol 1.’  Not so fast, friends, it now appears the Mars Phoenix project will start the clock at ‘Sol 0’ to continue the confusion.

We wouldn’t even be debating the niceties of astronomical anniversaries ‘tosol’ had the Opportunity and Spirit missions suffered the fate of the Mars Observer, Climate Orbiter, and Polar Lander NASA debacles. JPL lost contact with all three of those spacecrafts before a single experiment had been run or drop of data transmitted.  In the case of the Climate Orbiter, the problem was blamed on one engineering team using English units and another metric.  Hopefully all these Mars/Earth clockwatching differences won’t spell disaster for the next generation of rocket scientist, though only ‘morrowsol’ will tell.

Opportunity certainly deserves some celebration after two solar years, considering its primary surface mission was planned to last a mere 90 sols.  Hopefully, the robot will not suffer roving in the shadow of its’ older sibling Spirit that had already reached Sol 22 when Opportunity was a wee one Sol rookie rolling around the rocks.  Now that both mobile machines have passed the 1000 sol milestone (November 17, 2006) the rivalry between the vehicles has settled, but Spirit still refused to show up for Opportunity’s party at Cape Verde in the Victoria Crater near Duck Bay, choosing instead to wander between sites on the other side of the planet in an area known as ‘Home Plate.’  It is unclear if Spirit had hoped to receive transmissions of the Red Sox in the Earth World Series.

So now that we have all gotten comfortable throwing ‘sols’ around in our space talk, we should mention some of the variations in marking months and logging years on the Martian calendar.  Someday the planet will have Wi-fi and will need a system to make drop menu choices to fill out surveys and enter porn sites.  Apparently most astronomers prefer “Julian dates” where everyone agrees on a single “epoch” and starts counting upwards. 

Here on earth that number is currently 2,454,403, but who really wants to use an integer for newspapers and especially birthdays.  Naturally, the Martian solar year is going to need some extra months to even out that 687 sol year and a county courthouse full of juries is still out on that one.  Among the many neat name suggestions I found are “Sagan,” “Titars,” “Ascraeus,” and “Qzeta.”

The truly anal annual analyzer should also familiarize themselves with the differences between the sidereal year, tropical year, and March equinox year before closing the book on this subject, as I will now. Please don’t forget the 93,000 Martian year precession cycle of the planet’s rotational axis.  The rest of us can join together in singing “Happy Birthday” to the intrepid Opportunity as we share a marvelous high resolution photo from his party.

译文: 火星时间还是地球时间

什么是日期？如果没有历法，日期是否还会存在？如果人类的手不去拨动表针，时间是否还会流逝？