# What constitutes a "Sol" cycle in the film "The Martian"

In the film (and book) The Martian, NASA has termed a cycle of time as a "Sol", and it is what Watney and the crew use to refer to what seems like it might be a Martian day.

1. Is a Sol equal to one Martian day?
2. If so, is it explained why do they call it a Sol instead of a Martian Day, or maybe Martian Solar Cycle? (Could it be a cool-sounding abbreviation?)
• VTC as off topic, as this is straight up a question of science fact: en.wikipedia.org/wiki/Timekeeping_on_Mars#Sols Oct 8, 2015 at 4:59
• Copy the question and the answer to the appropriate stack, please. It applies to both the fictional world and the real world, but it still completely applies to the fictional world of The Martian. Oct 8, 2015 at 6:53
• the question is about a term used in a fictional work. it's on-topic on both stacks as OP was apparently not aware that it was a real-life term. Oct 8, 2015 at 10:50
• @phantom42 No more no-topic than questions about how thrust vector calculations can be used to maneuver space vehicles... (off topic here, on topic on physics SE). Oct 9, 2015 at 1:37

While the book uses the term, it does not explain the definition or exact length.

Real-life NASA, however, does.

One sol = one solar day.

Mars Solar Days and 24-hr Clock Convention

Following the long-standing practice originally adopted in 1976 by the Viking Lander missions, the daily variation of Mars solar time is reckoned in terms of a "24-hour" clock, representing a 24-part division of the planet's solar day, along with the traditional sexagesimal subdivisions of 60 minutes and 60 seconds. A Mars solar day has a mean period of 24 hours 39 minutes 35.244 seconds, and is customarily referred to as a "sol" in order to distinguish this from the roughly 3% shorter solar day on Earth. The Mars sidereal day, as measured with respect to the fixed stars, is 24h 37m 22.663s, as compared with 23h 56m 04.0905s for Earth.

• So a Sol is specifically for Mars and not for any other planet? It is not a general term for a solar day on a random planet? Oct 8, 2015 at 14:04
• @VincentAdvocaat You have it, at least in theory. It is the term for a solar day for a planet, and the context should make which planet clear. As a practical matter, it would only apply to Mars. Mercury is tidally-locked to the Sun, and therefore has no solar day. Venus rotates so slowly that it has just less than two sols per Venusian year. Jupiter, Saturn, Uranus, and Neptune are gas giants, and don't have definitely measurable solar days. Pluto isn't a planet anymore, but it does have a solar day. Would that be shortened to "sol" if/when we start operating there? We'll have to see. Oct 8, 2015 at 14:53
• @MontyHarder Mercury is not tidally-locked, that's known since 1965. en.wikipedia.org/wiki/Mercury_%28planet%29 Oct 8, 2015 at 17:10
• @edc65 I stand corrected. Mercury goes in the same category as Venus, only worse: With a 3:2 resonance, a solar day is two (Mercury) years, so it's hardly a subsidiary unit in the way days and sols are to Earth's and Mars' respective years. Oct 8, 2015 at 18:05
• About "sol" being used for other planets: I think it just hasn't come up yet, so "nobody knows if we will use it elsewhere". As a practical matter, Mars is the only body (so far) where we have landed anything, where we have had continuous extended operations on the surface, and where the day/night cycle is highly relevant for operational reasons (illumination, temperature, power, etc.), which leads to the need for a short, unambiguous name for the local solar cycle. I personally favor leaving it a Mars term, but I'm guessing it will be adapted eventually to other bodies, as needed. Oct 10, 2015 at 3:32

A sol is one solar day on Mars, which is a little longer than the 24 hour solar day on Earth. For some interesting perspectives on what it's like to work on the Sol schedule, check out this recent blog post: https://sketchtogether.com/blog/what-the-martian-didnt-show-you-how-scientists-communicate-with-mars.html

Had the term been something longer like "Martian day" it would inevitably get shortened or abbreviated at least sometimes, likely to "day" which could cause confusion.

To answer the question "why not Martian Day", the answer is simply for ease of use / to reduce confusion.

"Sol" was coined for the use of real-life teams operating probes and rovers on Mars. As in The Martian, most stuff is solar-powered, so operational schedules depend on Martian sunrise and sunset. They probably would anyway, for reasons of visibility and temperature. If the term was "Martian day" then people would be tempted to abbreviate it to "day", and then there would be perpetual confusion what kind of "day" anyone is talking about, which could lead to costly mistakes. A word that's distinct from "day" and easy to use in conversation solves that problem. And "sol" is as good as any other.

A Mars solar day has a mean period of 24 hours 39 minutes 35.244 seconds, and is customarily referred to as a "sol" in order to distinguish this from the roughly 3% shorter solar day on Earth.
http://www.giss.nasa.gov/tools/mars24/help/notes.html