All Astronautical Evolution posts in 2015:
“Drowning in Process” (Nov.)
SETI and Sanity (Oct.)
SpaceX, SpaceY, SpaceZ (Sept.)
Should We Phone ET? (March)
More Pluto Controversy (Feb.)
The Pluto Controversy (Jan.)
New in 2020:
2022: What’s to do on Mars?…
2021: New space company Planetopolis…
2020: Cruising in Space…
2019: The Doomsday Fallacy, SpaceX successes…
2018: I, Starship, atheism versus religion, the Copernican principle…
2017: Mars, Supercivilisations, METI…
2016: Stragegic goal for manned spaceflight…
2015: The Pluto Controversy, Mars, SETI…
2014: Skylon, the Great Space Debate, exponential growth, the Fermi “paradox”…
2013: Manned spaceflight, sustainability, the Singularity, Voyager 1, philosophy, ET…
2012: Bulgakov vs. Clarke, starships, the Doomsday Argument…
2011: Manned spaceflight, evolution, worldships, battle for the future…
2010: Views on progress, the Great Sociology Dust-Up…
Index to essays – including:
The Great Sociology Debate (2011)
Building Selenopolis (2008)
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Definition of annum 0
The martian vernal equinox on 5 September 1964 defines the start of the annum in which Mariner IV made the first close-up observations of Mars. This is Mariner Anniversary annum 0, or MA 0.
The following annum began with the equinox on 24 July 1966, and contains the first anniversary of Mariner IV’s flight. It is therefore Mariner Anniversary annum 1, or MA 1.
Date conversion method
Calendar dates are converted, firstly, into the Modified Julian Date (MJD) on Earth, or the Mars Sol Date (MSD) on Mars. A simple formula converts between MJD and MSD. The result may then be turned back into a calendar date.
MJD = Modified Julian Date:
The Modified Julian Day (MJD) is an abbreviated version of the old Julian Day (JD) dating method which has been in use for centuries by astronomers, geophysicists, chronologers, and others who needed to have an unambiguous dating system based on continuing day counts.
Start of the JD count is from 0 at 12 noon 1 JAN -4712 (4713 BC), Julian proleptic calendar. Note that this day count conforms with the astronomical convention starting the day at noon, in contrast with the civil practice where the day starts with midnight (in popular use the belief is widespread that the day ends with midnight, but this is not the proper scientific use).
The Modified Julian Day, on the other hand, was introduced by space scientists in the late 1950’s. It is defined as
MJD = JD – 2400000.5
The half day is subtracted so that the day starts at midnight in conformance with civil time reckoning. This MJD has been sanctioned by various international commissions such as IAU, CCIR, and others who recommend it as a decimal day count which is independent of the civil calendar in use.
(U.S. Naval Observatory)
MSD = Mars Sol Date:
This represents a sequential count of Mars solar days elapsed since 1873 December 29 at approximately Greenwich noon (Julian Date 2405522.0). This epoch was prior to the great 1877 perihelic opposition of Mars and precedes nearly all detailed observations of temporal changes on the planet. It corresponds to a Mars Ls of 277°, approximately the same planetocentric solar longitude as that for the Earth on the same date. MSD 44796.0 is approximately coincident with 2000 January 6.0, at a near-coincidence of prime meridian midnights on the two planets and a repetition of Mars Ls = 277°. The period 44796 sols also represents a near commensurability of 126 Julian years and 67 Mars tropical revolutions.
A MSD which is a whole number corresponds to midnight on the martian prime meridian.
Thus 6 January 2000 at 00:00:00 corresponds to MJD 51549.00, MSD 44796.00.
The conversion formula is:
|MSD – 44796 = (MJD – 51549) / 1.02749125|
The NASA Goddard Mars24 software gives MSD 32235.62 at the moment of the vernal equinox which we have chosen to begin MA 0. Thus the starting-point for the MA calendar, 00:00:00 AMT on 1 Gemini MA 0, is MSD 32235.00. From this we can work forwards and backwards using the formula for long and short years described earlier to find the exact sols which begin subsequent annums.
The exact moment of the vernal equinox will not necessarily fall on 1 Gemini each annum, any more than the vernal equinox on Earth falls on 21 March each year. But since the equinox in this particular annum falls fairly close to the middle of the sol, and since long and short years are interspersed at frequent intervals, we may expect that most of the time the equinox will indeed fall on this date.
Similarly, the southward equinox of that annum occurs (so far as one can tell from Mars24) around 08:00 AMT on 22 Sept. 1965, sufficiently far into the sol that variation of a few hours each way will not usually prevent it from coinciding with 1 Sagittarius.
The following three tables convert between the start of Mars annums and MSD numbers, between the start of Earth years and MJD numbers, and between calendar dates within the annum or the year and numbers of sols/days to obtain a precise MSD or MJD.
Table 1: the Mars Sol Date values for the start of Mariner Anniversary annums
|MA annum||Length of annum||MSD of 1 Gemini
|0||Long (669 sols)||32235.00|
|2||Short (66 sols)||33573.00|
Table 2: the Modified Julian Date values for the start of Earth years
The number of days in a decade = 3650 + the number of leap years in that decade (counting year 0 to year 9), which is 3 if the decade begins year 20, 40, 60, 80, or 2 if it begins year 10, 30, 50, 70, 90, 00 (except for year 2000, which was a leap year).
|Earth year||MJD of 1 January
|Earth year||MJD of 1 January
Table 3: MSD/MJD values within an annum/year
In order to find the MSD or MJD for any sol/day other than 1 Gemini/1 January, the number of 24-hour sols/days elapsed since the start of the year must be added. This is the sol of the annum/day of the year, minus one. All whole number dates refer to the beginning of the day or sol at midnight.
|MA calendar date||Elapsed sols||Earth calendar date||Elapsed days|
|1 Gemini||0||1 January||0|
|1 Cancer||63||1 February||31|
|1 Leo||126||1 March||59/60|
|1 Virgo||189||1 April||90/91|
|1 Libra||252||1 May||120/121|
|1 Scorpio||315||1 June||151/152|
|1 Sagittarius||371||1 July||181/182|
|1 Capricornus||420||1 August||212/213|
|1 Aquarius||469||1 September||243/244|
|1 Pisces||514/515||1 October||273/274|
|1 Aries||563/564||1 November||304/305|
|1 Taurus||612/613||1 December||334/335|
Example of date conversion
I am writing this on 16 April 2015 at about 09:30 GMT. From Table 3, this gives day 90 + (16 – 1) + 0.40 = 105.4 of the year.
From Table 2, 1 January 2010 was MJD 55197.00, therefore 1 January 2015 was 55197 + (5 × 365) + 1 = 57023. Adding 105.4 makes today MJD 57128.4.
Using the conversion formula produces MSD 50226.12.
From Table 1, MA annum 26 (a short annum) began on MSD 49619, which subtracted from 50226 leaves 607 sols over. Returning to Table 3, the start of 1 Aries is sol 563, leaving 44 sols over.
The martian date and time are therefore 44 Aries MA 26, at about 02:53 AMT. An accuracy of +/–0.01 sol/day would convert to about +/– a quarter of an hour.
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