
Mars Needs Leap Days Too - Hooke
https://www.nytimes.com/2020/02/28/science/leap-year.html
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wcoenen
> _That means that when the clock strikes midnight on New Year’s Eve, Earth
> hasn’t quite circled all the way back to its starting point._

Circling back to the starting point would be a sidereal year, which isn't what
we want either.

What the calendar is trying to synchronize to is not the sidereal year, but
the tropical year: the equinoxes and solstices should occur around the same
date each year, and the seasons shouldn't shift relative to the calendar over
the years. Because of the precession of the earth's axis, this tropical year
is about 20 minutes shorter than the sidereal year.

~~~
im3w1l
Why, physically speaking, is the tropical year almost but not quite as long as
the sidereal year?

~~~
wcoenen
The seasons are caused by the orientation of the Earth's tilted axis relative
to the sun. E.g the north pole is tilted towards the sun during summer of the
northern hemisphere, and away from the sun in winter.

Now imagine the Earth being magically held in place, instead of orbiting
around the sun. This would _almost_ stop the seasons from advancing, but not
entirely. There would still be a slow 26000 year cycle of the seasons, because
the Earth's axis precesses much like a spinning top. It's this effect that
speeds up the cycle of the seasons by about 20 minutes each year. (1
year/26000 is about 20 minutes)

~~~
im3w1l
Is there a reason it precesses at that particular speed, some resonance or
tidal locking or something? Or is it basically a random initial condition?

~~~
wcoenen
It's pretty random. It's mostly caused by gravitation from the sun and moon,
and counteracted by the Earth's spin; faster spin would mean slower
precession.

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JoeAltmaier
A spacefaring civilization wouldn't care much at all about planetary motion
and seasons. They'd likely go 'metric' with time.

"See you in a kilosec!"

"Oh she got promoted a couple megasecs ago, transferred out."

"I've been up for a hectasec; I have to catch some Z's!"\

"I graduated early, only 5 gigasecs old!"

~~~
btilly
5 gigaseconds is about 150 years, so your "graduated early" indicates some
serious life extension technology.

~~~
adeeds
or relativistic travel

~~~
sudhirj
Whose gigasecs, then?

~~~
jcims
Tinder for nerds?

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chrononaut
Somewhat related to this, back in 2004 the JPL team working on Spirit and
Opportunity were given specially-designed wristwatches that matched the 24
hour and 39 minute day on Mars in order to better match the daily working
schedule that Mars required.[0]

[0]
[https://mars.jpl.nasa.gov/mer/spotlight/spirit/a3_20040108.h...](https://mars.jpl.nasa.gov/mer/spotlight/spirit/a3_20040108.html)

~~~
lxmorj
I bet those watches sell for a pretty penny

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labster
[https://en.m.wikipedia.org/wiki/Timekeeping_on_Mars](https://en.m.wikipedia.org/wiki/Timekeeping_on_Mars)

I think you’ll want more detail than the article gave.

~~~
jolmg
> The term sol is used by planetary scientists to refer to the duration of a
> solar day on Mars. The term was adopted during the Viking project in order
> to avoid confusion with an Earth day. By inference, Mars' "solar hour" is
> 1/24 of a sol, and a solar minute 1/60 of a solar hour.

I wish they would have picked another name. As a Spanish speaker, I can't stop
my mind translating "sol" to "sun". I can already imagine hearing someone say,
"how was your sun today?"

~~~
samatman
Which is perfect, by analogy with month and moon.

If you spoke a language where the word for sun and day were the same, you'd
barely notice.

~~~
bobbylarrybobby
Chinese is one such language. Both “sky” and “sun” are also used for “day”

~~~
samatman
Yep!

Ming (明) is my favorite Hanzi, thanks to a lovely weekend spent at Sun-Moon
Lake in Taiwan.

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skizm
I assume there are people at NASA / SpaceX/ etc. in charge of writing custom
MarsDate modules for various languages. Sounds horrible. I'm not even sure
we've solved how to deal with dates on earth yet.

~~~
ShinTakuya
I'd be more interested to see if a more generalised PlanetDate module could be
written. Perhaps outside of Earth we could do away with months and just stick
to days (based on rotations, assuming the planet rotates) and years, with leap
days added in between? And if the planet doesn't rotate, just count by hours
and years (with hours having a logical separator at 10 or 100)?

~~~
dahfizz
I think any interplanetary civilization will be practically forced to use a
planet-agnostic time system - a true UTC

~~~
wahern
That'd be
TCB--[https://en.wikipedia.org/wiki/Barycentric_Coordinate_Time](https://en.wikipedia.org/wiki/Barycentric_Coordinate_Time)

But we'd need a different standard if we colonized planets outside our solar
system.

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jedberg
“There may come a time — if you have cultural civilizations that are living in
this environment and you want to preserve the seasonal significance of a
particular date on the calendar — that you would likely introduce some sort of
a leap system,”

It never occurred to me until reading this that the main reason we care about
syncing our calendar to the seasons is mostly cultural.

We could easily just count days and have equal months and be done with it if
we didn't care about certain days happening in the same season.

Very interesting!

~~~
dahfizz
In fact, it was the Catholic church that officially declared the Gregorian
calendar we use today. The motivation was to make sure Easter always fell on
the same day.

~~~
btilly
Not quite on the same day as may be seen that it didn't fall on the same day
before or after.

But he wanted it to stay close to the equinox.

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sudhirj
Doesn't every planet? I'd be astonished if every planet completed a revolution
in a perfect multiple of rotation time. Wouldn't it would have to be an
engineered system if it did.

Or is that just tidal locking? I know the moon does something similar but
there it seems like revolution time = rotation time, not perfect multiple.

~~~
twic
> But there is one planet where the calendar would need zero finessing:
> Mercury. The small planet revolves exactly three times, or days, over the
> course of two years — allowing its calendar to naturally align every other
> year.

I would imagine that this is some form of resonance, like the tidal locking of
the moon, but i can't explain it myself.

~~~
Sharlin
It is indeed:

> In some cases where the orbit is eccentric and the tidal effect is
> relatively weak, the smaller body may end up in a so-called spin–orbit
> resonance, rather than being tidally locked. Here, the ratio of the rotation
> period of a body to its own orbital period is some simple fraction different
> from 1:1. A well known case is the rotation of Mercury, which is locked to
> its own orbit around the Sun in a 3:2 resonance.

[https://en.wikipedia.org/wiki/Tidal_locking](https://en.wikipedia.org/wiki/Tidal_locking)

