Inner Planets on the Move

   This past month, November, the inner planets Mercury and Venus were very visible in the early morning skies before sunrise. Both of the planets were at or near their respective western elongation. Venus was there during August and Mercury reached its western elongation on November 10th.
   What I wanted to capture was the daily change in the position of Venus as it passed the star Spica in Virgo the Harvest Maiden. Venus moves about 1.6o each day so its eastward motion should be obvious after a day or so. It was – it is!. Mercury, if you are wondering, moves about 4o each day.
   Both inner planets were somewhere around their maximum separation from the Sun – as we see it the inner planet is to the right side of the Sun, or toward the west.
   The series of pictures were taken from two locations near my house. One is from an empty lot near U.S. Highway 50 looking east. The other pictures are from somewhere along the street I live on! The first picture was taken at Legacy Park and includes the ISS orbiting over my location.


   
   
   

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Mercury at Western Elongation – See it in the Morning Skies

   On Tuesday November 10th Mercury, the innermost planet, will reach its orbital position known as greatest western elongation at 19.1o. At that moment Mercury, the Sun, and the Earth, would be arranged in something close to approximating a right angle as this graphic shows. Even though it sounds confusing at western elongation for either Mercury or Venus the inner planet will be to the right of the Sun as we view them, meaning that at western elongation an inner planet rises in the east before the Sun rises. And at eastern elongation with the inner planet on the left side of the Sun the inner planet follows the Sun across the sky setting after the Sun sets.

   From our perspective the orbits of Mercury and Venus appear to move from one side of the Sun to the other – out to the left (east) from the Sun to eastern elongation, then reverse and move westward (inferior conjunction) between the Earth and the Sun to western elongation. From there the inner planet moves eastward going behind the Sun (superior conjunction) and eventually reappearing on the eastern side of the Sun for an eastern elongation. Repeat over and over – do not stop!

   There is a lot to see and a few things you cannot see on the morning of Mercury’s western elongation. Going by relative apparent magnitudes Mercury (-0.52), Venus (-3.99), Spica (0.96), are all easily visible as bright to very bright star-like objects. The 24-day old waning crescent Moon shines at a much brighter apparent magnitude of (-11.52).

   
   
   
   
   

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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Venus at Western Elongation

inner-planets-positions   Wednesday August 12th the inner planet 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 a few other planets, Jupiter, Uranus, Mars, and Dwarf Planet Ceres. The waning crescent Moon is nearly straight overhead (at least from 40oN) and a few degrees from the open star cluster the Pleiades. In this graphic the orbital path of Venus has been added, and also an arrow to show the direction Venus will now be moving toward (eastward).

   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 earliest time for this orbit. Locally Venus rises that morning at around 3:00 am CDT. On the day of the western elongation Venus will be 45.8o 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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Solar Opposites

   Inner planets like all the planets as viewed from Earth appear to be moving from one side of the Sun to the other. As a result the visible planets are seen either in the morning skies when located west from the Sun and rising before the Sun rises, or in the evening skies east from the Sun, and setting after the Sun sets. For the two inner planets, Mercury and Venus, these positions on either side of the Sun are known as elongations – western or greatest western elongation and its counterpart, eastern or greatest eastern elongation.
   As the graphics below show, both inner planets reach their respective elongations – but on opposite sides of the sun. Monday March 23rd Mercury is at western elongation in the morning skies, and on Tuesday March 24th Venus will be at eastern elongation in the evening skies.

   
   
   

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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