词条 远日点
描述: 远日点是绕太阳公转轨道上距太阳最远的点。绕太阳运行且不受任何其他天体影响的天体，其轨道呈椭圆形，太阳位于该椭圆的焦点之一。在数学上，远日点标志着椭圆主轴的一端。在这个词中，”ap“表示最远点，”heliion“表示太阳。因此，只有当中心天体是太阳时才可以使用该术语。当中心天体是一颗非太阳恒星时，该术语为"apastron"或"apoastron"；当绕行的中心天体是地球时，该术语为"apogee"。无论中心体是什么，一般的术语都是”apoapsis”。
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Halley's Comet
图注： Halley’s Comet, is a well-known periodic comet, named after the English astronomer Edmond Halley. It has an orbital period of approximately 75 years and is visible from Earth with the naked eye when it passes through the inner solar system.
The image shows Halley’s Comet, officially designated 1P/Halley, with a tail of gas and dust streaming away from the Sun. It was taken from the La-Silla-Observatory in Chile in 1986 during Halley's Comet's last visit to the inner solar system. The stars in this image appear elongated or as lines of three different colored dots as image was created from three separate observations in different colors of light and the telescope was tracking the comet, which was moving very slightly compared to the background stars. Note that the comets tail does not point in exactly the same direction as the elongation of the stars. This shows us that the comet tail is not always behind the comet, but instead pointing away from the Sun.
The comet passes its perihelion (closest distance to the Sun) at a distance of around 0.59 astronomical units, right between the orbits of Mercury and Venus. Beyond Neptune, it reaches its aphelion (furthest distance from the Sun) at a distance of approximately 35 astronomical units. Halley's Comet reached aphelion in December 2023 and is now moving inwards again. It is expected to be seen with the naked eye from Earth again in mid-2061.
来源： ESO
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