As the year goes on, the angle of the earth changes. Not only do we rotate once every 24 hours, and orbit Sol once every 356 days ( more or less ) but the angle of inclination of our planet shifts. Hence, the seasons as the hemispheres are slightly more or less exposed to the sunlight.
Here’s the question: As the earth slowly tilts, do man-made satellites stay where they are, or do they actually shift in their orbit as well to match the tilt in the poles ? If they do NOT tilt on their own, is each individual satellite moved to keep on track?
And, most interestingly to me, if they DO stay where they are, how do they do this? Once they are in geosynchronous orbit around the earth, are they held to that rotational position by gravity? Why don’t they keep orbiting through space, irregardless of the tilting of the earth’s axis?
The Earth’s tilt doesn’t change (well, not much–it varies between about 21 and 24 degrees over a 41,000-year cycle, give or take). The tilt with respect to the Sun varies and causes seasons, but this has zero effect on the orbits of satellites, which orbit with respect to the Earth.
The angle of inclination of the earth’s axis does NOT shift. It maintains a constant 23.5 degree tilt at all times, with the axis pointing roughly at the Pole Star. What alters is the direction of the sun as seen from the earth. When the sun is in the direction of the axial tilt we have summer (or winter); when it is at right angles to the direction of tilt we have spring (or autumn).
Since the earth’s axis doesn’t change in direction or tilt over the course of the year, the question of satellites having to “shift” doesn’t arise.
And to further that, with the exception of some communication satellites, most satellites are not in geosychronous or geostationary orbits. The constellation of GPS satelllites (I think there are 27 in orbit right now) are in a complex ballet of almost circular orbits that guarentee that at least 3 (almost always 4) will be over the horizon for any point in the Earth, but their orbits lag behind the Earth’s rotation such that if you were to trace the projection of their trajectories down on the Earth it would like like a ball of yarn. These are called subsynchronous orbits, or among the technical elite, “ball of yarn” orbits.
There are also other highly elliptical orbits like Molniya orbits and tundra orbits which are intended to permit long dwell times over high latitudes that would otherwise not be possible with near-circular orbits. These can be geosynchronous or not, depending on whether the orbital period is some multiple of the Earth’s rotational period. Most survellience satellites have orbits like these, obviously to permit maximum dwell time over target.
Satellite orbits can behave in surprising ways as viewed from Earth. The Sirius Radio satellites as seen from the US come shooting in from the southwest, hang around in the western sky for a while, then go shooting off to the southeast (I might have this reversed but the point is the same).
But all satellite orbits are ellipses having one focus at the center of the Earth, and they change very little over time with respect to the distant stars (though not with respect to the Sun and Moon).
Does the wobble still allow the Earth to always maintain the 23.5 degree angle with respect to the ecliptic? Do the other long-term cycle effects on the Earth’s orbit change that over time?
It seems like these time cycles are on far too long a scale to effect the orbit of satellites, but if we had some hypothetical satellite designed to maintain position with respect to some point on the Earth’s surface for 100,000 years, would we need to worry about the longer term changes in the Earth’s axis?
No. I already addressed this. The angle of axial tilt wobbles between 22.1 and 24.5 degrees in a 41,000-year cycle. And yes, that’s too long to affect the the orbit of a typical satellite with a 10-year expected lifetime, give or take.
The orbital angle is roughly stable (within the precision of the stated value) over the near term. The angle will change over time (as stated above) but this has a virtually insignificant impact upon the orbit of the Earth.
No real-world satellite will stay in a stable Low Earth or Mid Earth orbit on anything like the order of 100k years. If it did, though, yes, you’d have to adjust for the Earth’s change inclination, else the orbital path (with respect to the Earth) would change.
Since the OP has been answered (there is virtually no change in the axis of the Earth’s rotation with respect to the fixed stars over the course of a year), I’d just add a couple of comments to extend the posts above.
The axis of rotation of the Earth is neither fixed absolutely with respect to the Earth nor is it fixed permanently with respect to inertial space.
Over the long-term, the Earth’s angular momentum vector changes direction due to outside torques (from the Moon and Sun) acting on the oblate Earth, causing precession - this is the change in the axis of rotation that sweeps out a circle with respect to the celestial sphere over ~26,000 years.
Over the short-term, the Earth’s angular momentum vector is nearly constant, but due to slight shifts in mass distribtion caused by gravitational attraction from the Moon, Sun, etc., the axis of rotation nutates - the largest component of the nutation has an 18.6 year period.
Yeah - I was confused because the very next post after yours said it didn’t change, so I wasn’t sure whether it just didn’t change at all, or if the change was a tiny one, as you specified.
