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I saw David Ratti's question about Yuri Gagarin's voyage on Vostak 1 and how it was considered orbital, and that it was considered the first.

I was wondering what constitutes one full trip around the Earth, it was only considered "orbital" because he reached a distance and speed where he could have stayed in "orbit" around the Earth.

How is one full Orbit of the Earth Measured?

Point of Reference from Earth or Point of Reference from a stationary orbit plotted around the Earth that is not affected by it's rotation?

I am probably using the wrong terminology, I apologize.

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    $\begingroup$ Obviously he did complete an orbit. Most of the comments in that other question are focused on what "orbital flight" means instead of what "an orbit" means. Gagarin orbited the Earth. $\endgroup$
    – David Hammen
    Oct 9 '14 at 23:18
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    $\begingroup$ He went along the rotation of Earth, not against. $\endgroup$
    – Anixx
    Oct 10 '14 at 14:53
  • $\begingroup$ I see that now. can I change the content of my question? I mean without changing the outlying question of How is one full Orbit of Earth Measured $\endgroup$
    – Malachi
    Oct 10 '14 at 15:01
  • $\begingroup$ Answer this: "How long does it take a geosynchronous satellite, on station, to complete "one full Orbit of the Earth", and you've answered the OP's question... $\endgroup$
    – DJohnM
    Oct 10 '14 at 16:39
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In the case of a Keplerian orbit, an orbit is a 360 degree change in true anomaly.

Suppose a spacecraft in low Earth orbit (LEO) passes directly over some point on the Earth. Where is that point relative to the spacecraft one orbit later? I'll use 90 minutes, or 1/16 days, as the orbital period for our spacecraft. The Earth rotates to the east by 22.5 degrees during that 90 minute orbit. Thus the point over which the spacecraft passed one orbit ago is now 22.5 degrees to the east of the spacecraft's sub-satellite point. Alternatively, the spacecraft ground track shifts 22.5 degrees to the west per orbit.

To illustrate this, the image below portrays the ground track for the International Space Station over two orbits. This clearly depicts that the ground track shifts by about 22.5 degrees to the west per orbit.

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    $\begingroup$ Notably, this has no relation to the motion or surface of the body being orbited, so it doesn't matter how that appears vs surface coordinates. $\endgroup$
    – Phil H
    Oct 10 '14 at 10:11
  • $\begingroup$ could you explain this a little better for me, my initial thought is the he went east, Earth rotated west, so he would have went less than 360 around Earth, is this true? (he = Yuri) $\endgroup$
    – Malachi
    Oct 10 '14 at 13:15
  • $\begingroup$ The Earth rotates to the east, not the west. Suppose a spacecraft in low Earth orbit (LEO) passes directly over some point on the Earth. Where is that point relative to the spacecraft one orbit later? I'll use 90 minutes, or 1/16 days, as the orbital period for our spacecraft. The Earth rotates under the spacecraft by 22.5 degrees during that 90 minute orbit. Thus the point over which the spacecraft passed one orbit ago is now 22.5 degrees to the east of the spacecraft's sub-satellite point. Alternately, the spacecraft ground track has moved 22.5 degrees to the west. $\endgroup$
    – David Hammen
    Oct 10 '14 at 14:01
  • $\begingroup$ I'll add the above to my answer. $\endgroup$
    – David Hammen
    Oct 10 '14 at 14:05
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One approach is to consider the orbit in an inertial frame, disregarding the position with respect to the surface of the Earth (and the Earth's rotation). Then the period is $$ T = 2 \pi \sqrt{\frac{a^3}{\mu}} $$

Where $T$ is the period, $a$ is the semimajor axis and $\mu$ is the gravitational parameter (the universal gravitational constant multiplied by the mass of the central body).

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