I'm specifically interested in knowing how long they take to do a full circle around the earth but don't find any info. Could anyone help me?


First check the altitude of the satellites. New ones deploy around 300 km and will then climb to about 500 km. Altitude is a small effect because what matters is the radius of the orbit; the distance to the center of the Earth.

When orbit altitudes are quoted they are usually referenced to the equatorial radius of the Earth even if they are not equatorial orbits (think "maximum headroom" (see answers to How is the altitude of a satellite defined, given that the Earth is not spherical? for example).

We can assume the orbits are circular, and add 6378 km to the altitude to get the orbit's semimajor axis $a$. For 300 km and 500 km it will then be 6678 and 6878 km respectively.

You can get the velocity from the vis-viva equation

$$ v = \sqrt{\frac{GM}{a}}$$

You can look up $G$ and $M$ separately or find the product $GM$ for Earth in a table of standard gravitational parameters where we can see that Earth's is 3.986E+14 m^3/s^2. Converting the semimajor axis values to meters (be careful of units!) we get velocities of 7726 and 7613 m/s.

The period of a circular orbit is the circumference divided by the velocity:

$$T = 2 \pi a / v,$$

put the two equations together and you get

$$T = 2\pi \sqrt{a^3/GM}$$

which works for elliptical orbits as well.

So we get 5431 and 5677 seconds, or 90.5 and 94.6 minutes.

  • 2
    $\begingroup$ I'm amused that my initial rough guess, 90-95 minutes, was so close to your carefully calculated guess. $\endgroup$
    – PearsonArtPhoto
    Feb 3 '20 at 0:04
  • 1
    $\begingroup$ @PearsonArtPhoto I'm never amused by authoritative sounding yet unsupported Stack Exchange answers. Support is what makes the difference between a comment and an answer in SE. $\endgroup$
    – uhoh
    Feb 3 '20 at 0:35
  • $\begingroup$ I did check the 95 minutes, I just rounded up. But I agree, yours is a better answer. $\endgroup$
    – PearsonArtPhoto
    Feb 3 '20 at 0:59

They orbit around the Earth about once every 90 minutes or so. The one that I looked was 95 minutes, but they vary somewhat by altitude. They all will be between 90-100 minutes, and probably an even more narrow range.


While uhoh did the standard basic math, You can check public TLE data for almost all satellites, which contains an approximate orbital period. For Starlink satellites, you can find info here. So you'll see that the (approximate) periods are between 89.73 and 91.53 minutes as of 03Feb2020.

The reason for the "approximate" period, is that first, due to perturbations, there will always be slight variations between times of crossing a reference point on orbit, and also, those are also based on observation-estimated orbits, which also have a limited accuracy of their own, but you might expect estimations as such to be accurate up to 1 second.


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