Sky Object Elongations - SkyMarvels.com

MAJOR SKY OBJECT ELONGATIONS 2021

The scrollable Sky Object Elongation diagram above is one of the quickest ways to learn what is happening in the sky for all of 2021. In a glance it reveals the elongations of the major sky objects—that is, how far east and west of the Sun they appear—throughout the year. This lets you easily de- termine whether a particular object will rise this morning before the Sun, or set this evening after it!

For the Inferior Planets the diagram highlights their zigzag motions that we see from Earth, as well as their Greatest Elongations for the year ( Mercury, Venus ). For the Moon, it indicates the lunar motion, which is continually eastward (leftward in the diagram), as well as Eclipse Seasons, their eclipses, and major lunar phases. New Moons are shown only when they produce the two solar eclipses in 2021! For all other major sky objects, we see their generally westward motion (right- ward in the diagram) relative to the Sun, as Earth proceeds in its orbit.

Keep in mind that this is not a map of the entire sky. Each "horizontal strip" of the diagram indicates only the sky objects' east-west distances from the Sun on a particular date; it does not indicate their north-south distances. In the vast majority of cases, wherever paths "seem to cross" the two objects involved may be (and most often are) separated by some north-south distance. This distinction is important, especially where the paths of Mercury and Venus appear to cross that of the Sun. In the diagram each such crossing does not signal a transit because, from our vantage point on Earth, Mercury and Venus most often "pass over or under" the Sun when we see them move past it. In fact, transits of

the Sun by the Inferior Planets are so rare that Mercury's next transit is in 2032 and Venus's not until 2117!

Even so, where two paths in the diagram cross, the objects involved do share the same elongation, and likewise the same Right Ascension, so they are said to be "in conjunc- tion". In everyday terms this is when they are at or nearly at their closest distance from one another. The diagram above will verfy this List of Conjunctions, many sky events on This Calendar, and our pages highlighting the best Moon-Planet Conjunctions and Planet-Planet Conjunctions. And when they occur, view them on our Current Sky page.

You'll note that—in terms of east and west—the elongation diagram always shows 360° of the sky. However, you can scroll your view, depending on how you want to look at the heavens. For example, if you center the "middle vertical string" of Full Moons on the diagram, it is as if you are looking directly away from the Sun! That is, the Sun would be at your back! Or slide the Sun all the way to the left or right and imagine that it is rising or setting respectively! The various perspectives can be quite enlightening.

Note: the elongation diagram above is an enhancement—by SkyMarvels.com—of a previously existing original work, as permitted by and published here under the terms of the Creative Commons Attribution-Share Alike 4.0 License. The work on which this is based was created by and is available on the excellent http://armchairastronautics.blogspot.com/ website, whose efforts to further our understanding of the Universe are greatly appreciated.

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