# Tag Info

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Back in 1246 CE, the perihelion coincided with the December solstice. The Earth was at perihelion at 1 Jan in 2005 and will be on 6 Jan in 2096, GMT time. The drift is not monotonic, but it does progress overall throughout the centuries. Eventually, it will drift towards the March equinox and beyond, to come back to the winter solstice in around twenty ...

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Orbits in Schwarzschild spacetime can be described by the effective potential $$V_\text{eff} = -\frac{GM}{r} + \frac{\mathfrak{l}^2}{2r^2} - \frac{GM\mathfrak{l}^2}{c^2r^3}\text{,}$$ where $\mathfrak{l} = r^2\dot{\phi}$ is the specific angular momentum of the orbit, which is a conserved quantity. The first two terms match the form of the Newtonian effective ...

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Very cool question. I want to get into a little bit of detail here because otherwise there would be a one-paragraph answer, and I don't think that would cut it. So here goes. The planets in the solar systems have orbits with pretty low eccentricities (see this for more eccentricity values). At the upper end is Mercury, with an eccentricity of 0.2056. At the ...

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For really low orbits over an atmosphereless body, the body needs to have a uniform density. Otherwise the gravity field is not symmetric, the orbit changes shape over time, and you end up with a crater. NASA had trouble with this, lunar Mascons, during the Apollo era. In one case: "The Moon has no atmosphere to cause drag or heating on a spacecraft, so ...

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The two terms are used in answering different questions. Hill Sphere: given a large mass (eg Sun) and a small mass (eg Earth), can a tiny mass (eg Moon) find a stable orbit around the small mass? (If the tiny mass goes outside the Hill Sphere of the small mass, no.) SOI: given two large mass objects and a small object between them, (eg sending a probe ...

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The beginning of the year (1st of January) has been chosen for historical reasons (near the winter solstice) and has absolutely nothing to do with Earth's perihelion passage. Earth's perihelion could as well have occurred on the 10th of October. That it is near the 1st of January is pure chance (see for instance a post by EarthSky). It is, therefore, not a ...

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