On Saturday December 15th Mercury, the innermost planet, will reach its orbital position known as greatest western elongation at 21.3o. 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!
Mercury is visible in the morning skies just before sunrise local time, as this graphic shows. Venus, to the west from Mercury rises about 2 hours before Mercury, and Jupiter rises shortly after Mercury.
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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