Sunday January 6th Venus reaches the point in its orbit called greatest western elongation. As this graphic shows the inner planet Venus, or Mercury, is more or less at a right angle (90o) from the Sun and Earth at western elongation. From the surface of the Earth, your backyard, for example, Venus is to the right, or western side of the Sun and is rising before the Sun.
On the day of the elongation Venus will be very visible over the southeastern horizon at or before sunrise local time. Venus is joined by the planet Jupiter, about 13o below Venus, east from Venus. The reddish star Antares in Scorpius the Scorpion is also about 13o from Venus, and is about 6o from Jupiter. If the skies are dark enough the Milky Way should be visible rising toward the southwest.
At western elongation Venus, or for that matter Mercury the other inner planet, is as far out from the Sun as we see them and as a result Venus or Mercury will rise at the latest time in this orbit. On the day of the western elongation Venus will be 46.9o from the Sun. From western elongation forward Venus or Mercury will be moving eastward toward the Sun and each day rising closer and closer to the time of sunrise. As the planet moves eastward it is moving further away from the Earth toward superior conjunction on the opposite side of the Sun.
As the distance between the Earth and Venus, or Mercury, increases combined with the decreasing angle between the planet, the Earth, and the Sun, Venus or Mercury decreases in apparent size and also waxes through gibbous phase shapes but we never see it at a full phase since that is at superior conjunction.
Click here to go to the Qué tal in the Current Skies web site for monthly observing information, or here to return to bobs-spaces.
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