5
$\begingroup$

A recent non-answer to an unrelated question stated that the ISS crosses 16 time zones during New Year's Eve, which is obviously incorrect, the actual answer being "around 200".

But this inspired me to a follow-up: How many (minimum, maximum) time-zones does the ISS fly over during one full orbit? According to Wikipedia there are currently 38 time zones across the world, some large, some small. None of them is located entirely at high latitudes, so all can be visited by ISS at some point.

Let's define "one orbit" as the time between crossing one meridian and crossing the same meridian again. I guess that daylight savings time should be excluded for simplicity.

$\endgroup$
8
  • 1
    $\begingroup$ @geoffc - there are 38 time zones from UTC-12 to UTC+14. And it's not possible to touch all of them during one orbit. I guess the lower limit is below 24 - e.g. China is one huge zone and allows to "skip" some. $\endgroup$ – asdfex Dec 29 '20 at 17:11
  • 1
    $\begingroup$ @asdfex those are local political timezone choices, and not really relevant unless you want to change your question to "How many subsections of which countries does ISS fly over..." in which case it depends rather a lot on which orbit, given the changes in latitude during its travel. Same as for the moon. $\endgroup$ – Carl Witthoft Dec 29 '20 at 18:03
  • 2
    $\begingroup$ @CarlWitthoft What other kind of timezones exist? There is no such thing as "idealized time zones". Sure it depends on the exact orbit, that's why I asked for both the minimum and maximum. And also, it's absolutely clear that this question is not essential to space travel, but rather of the "useless trivia" kind. $\endgroup$ – asdfex Dec 29 '20 at 18:13
  • 1
    $\begingroup$ Crazy... you can cross the date line 6 times within one orbit... $\endgroup$ – asdfex Dec 29 '20 at 19:01
  • 2
    $\begingroup$ What would be ideal is the ISS's ground track overlaid on a time zone map. Plot enough ground tracks until an orbit repeats the first orbit. (This would approximately cover all possible orbits.) Based on each Earth longitude of the ascending node, count how many time zones are crossed. This would produce a table of ascending node longitude versus time zones crossed. Sounds like fun, but beyond my ability (and time availability, no pun intended). $\endgroup$ – JohnHoltz Dec 30 '20 at 16:52
4
$\begingroup$

Here are some results of me playing with a map and ground tracks. I took a plot of ISS ground tracks and overlaid it onto the most detailed map of time zones from Wikipedia.

First and foremost, time zones are weird. Even if we take "regular" time zones only and don't look at any of the political decisions, Earth has 25 time zones from UTC+12 to UTC-12. On top of that, there are 13 further differing time zones with times like UTC+12¾ on the Chatham Islands.

None of these zones is placed further North or South than 51° latitude in its entirety, i.e. all 38 of them can be visited by ISS in one orbit or another.

To start off with a curiosity: You can cross the international data line at least 6 times (possibly 8 if we trust the drawing on the map to be precise) during a single orbit:

To get a minimum number of time zones to cross, we have to look for possibilities to "skip" the regular ones. E.g. we could skip

  • UTC+0 by passing over Western Sahara
  • UTC+4 over the right part of Russia
  • UTC+5 over Iran (although at the cost of taking Irans own UTC+3.5 zone)
  • UTC+7 over China
  • UTC-4 over Argentina

Others like UTC+2 can't be skipped by ISS because Ukraine extends further North than the 51° inclined orbit reaches.

For some other time zones the map looks like they could be skipped, but these happen to be over international waters where the default time zone based on 15° segments applies.

Actually it seems possible to skip two of them in the same orbit, totaling to 22 time zones crossed: 22 Zones

The maximum of zones can be visited by including as many of the special time zones as possible, e.g. UTC+5¾ of Nepal or UTC+14 of Kiribati. Judging the exact amount is difficult from the map, because zones around islands are shown larger than they actually are. Anyhow, I was able to find an unambiguous 32 time zone orbit that doesn't make use of islands: 32 zones

$\endgroup$
2
  • $\begingroup$ +1 for "First and foremost, time zones are weird" if nothing else $\endgroup$ – Organic Marble Jan 2 at 18:38
  • $\begingroup$ And another for the "playing with a map and ground tracks" methodology. I can't think of anything else that would be better. $\endgroup$ – David C. Rankin Jan 3 at 18:08
-2
$\begingroup$

No, you should not define "one orbit" that way. Any orbiting object completes one orbit in the time it takes to go once around the planet in inertial space, irrespective of how fast the meridians of longitude drawn on the planet or other body being orbited are rotating as it does so. Instead, base your notion of one orbit on how often the ISS or other satellite passes through the plane of the equator from south to north (the "ascending node" of the orbit). The higher the altitude of the orbit, the fewer orbits are made during one day.

The ISS orbits at an altitude of about 420 km, so it takes about 93 minutes to make one revolution, during which time the earth has turned about 23 degrees in longitude. Since the ISS, like most other satellites, is going around in the same direction the earth is ("prograde" rotation), it flies through 383 degrees of latitude in each orbit.

This is exactly where it would make sense to discuss an idealized, apolitical, mathematical time zone. There are 360 degrees in a circle and 24 hours in a day, so an idealized timezone is a wedge-shaped section of the planet between two meridians 15 degrees apart. The ISS passes through 25.53 such idealized timezones in each 93-minute orbit.

GPS satellites orbit much higher, so they only go around twice per day, passing through 36 such timezones in each 11 hour, 58 minute orbit. Note that is not 12 hours, because they are orbiting once every sidereal day, not every solar day.

$\endgroup$
1
  • 4
    $\begingroup$ -1. For the purposes of OP's question, which is about actual time zones, which exist in the Earth's rotating frame, OP's definition of "one orbit" is much more useful than the usual one. $\endgroup$ – Russell Borogove Dec 31 '20 at 3:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.