This evening and for the next several the innermost planet Mercury will be visible at sunset above the southwest horizon as the banner graphic at the top of the page shows. Mercury reaches inferior conjunction, between the Sun and Earth, on the 15th and between now and then Mercury will be moving west toward the Sun. Each evening at the same time Mercury may be observed to be lower above the horizon as it moves toward inferior conjunction as this animated graphic is showing.
You might also notice that in each frame (day) the Sun is a little higher at the same time. This is the Sun’s apparent daily eastward motion along the ecliptic. This is also why it takes such a relatively short time for Mercury to orbit from eastern elongation to inferior conjunction. Combine the Sun’s nearly 1 degree/day east with Mercury’s westward orbital motion of 4 degrees/day and it is easy to see why the ‘closing rate’ is so fast.
Another interesting and observable change is Mercury waning from a small gibbous phase to a larger crescent phase as this small animated graphic is showing. It shows the planet from the 1st to the 15th when Mercury reaches inferior conjunction.
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.