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The ancient Greek astronomer Hipparchus noted the apsidal precession of the Moon's orbit (as the revolution of the Moon's apogee with a period of approximately 8.85 years); [4] it is corrected for in the Antikythera Mechanism (circa 80 BCE) (with the supposed value of 8.88 years per full cycle, correct to within 0.34% of current measurements). [5]
Earth's rotational velocity also varies in a phenomenon known as length-of-day variation. [171] Earth's annual orbit is elliptical rather than circular, and its closest approach to the Sun is called perihelion. In modern times, Earth's perihelion occurs around 3 January, and its aphelion around 4 July.
For Earth's current orbital eccentricity, incoming solar radiation varies by about 6.8%, while the distance from the Sun currently varies by only 3.4% (5.1 million km or 3.2 million mi or 0.034 au). [11] Perihelion presently occurs around 3 January, while aphelion is around 4 July.
Ignoring the influence of other Solar System bodies, Earth's orbit, also called Earth's revolution, is an ellipse with the Earth–Sun barycenter as one focus with a current eccentricity of 0.0167. Since this value is close to zero, the center of the orbit is relatively close to the center of the Sun (relative to the size of the orbit).
Being so close to the Sun, at perihelion the asteroid is moving at 106 km/s (240,000 mph). [5] The relativistic perihelion shift of this object is 1.6 times that of Mercury, which is 42.9 arcseconds per century. [8] With an observation arc over 4 years, the orbit quality of 2021 PH 27 is well secured, with an uncertainty parameter of 3. [4]
The Cambrigde Planetary Handbook, Cambridge Univ. Press, 2000, I find the distance from the sun to the perihelion is 147,085,800 km and the distance from the sun to the aphelion is 152,104,980 km; adding these gives a major axis of 299,190,780 km and so a semi-major axis of 149,595,390 km. Page 18 of the same book gives Earth's orbital ...
Biela's Comet was the subject of several panics over close approaches to Earth. An 1877 newspaper illustration from Chile, captioned "inevitable impact of the Earth with Comet Biela" Biela has sometimes been proposed as the source of meteoric impacts on Earth.
Although Mercury is farthest from Earth when it is full, the greater illuminated area that is visible and the opposition brightness surge more than compensates for the distance. [145] The opposite is true for Venus, which appears brightest when it is a crescent , because it is much closer to Earth than when gibbous .