A Tale of Two Phases

   Over the next couple of weeks the two inner planets, Mercury and Venus, will be above the eastern horizon in the hour or two before the Sun rises. Given its much brighter apparent magnitude (-3.92) Venus will be more noticeable. However Mercury (0.03) is only a few degrees away, down to the left as they both rise. This graphic shows the two planets (sizes greatly enlarged for the graphic) and their respective orbital paths. Both planets are moving eastward toward the Sun and will eventually catch up with and move to the opposite side of the Sun, behind the Sun as we view from Earth to their respective superior conjunction.

   As these two planet orbit toward the Sun they pass through phase changes much like our Moon. Currently both planets are waxing gibbous but as they move into superior conjunction they will be at full phase. But unlike our Moon at full phase the inner planets are not visible at their respective full phase. When they do reappear on the east side of the Sun they will be again at gibbous phase, and will be following the Sun across the sky, setting after the Sun as an ‘evening planet’.


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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