This month marks the beginning of somewhat planet-less evenings for a while. Since this past winter the two giant planets, Jupiter and Saturn have been very visible. Now they are moving toward conjunction with the sun, which as we view it from Earth, means that they are moving along their orbits and soon will be on the opposite side of the sun. However within a few months both planets will be visible in the east as they re-appear from behind the sun, and will be rising ahead of the sun.

While the absence of the two giant planets may certainly be missed the 'red planet' Mars slowly moves into the evening skies during this month. In fact it could almost be said that Mars backs into the evening skies as about midway through the month the regular eastward motion comes to a halt and then Mars starts moving in the opposite direction, backward toward the west. This apparent motion of Mars is called retrograde motion, and any object traveling or appearing to be traveling in an opposite or backward motion relative to the observer is said to be in retrograde motion

For an outer planet, such as Mars, retrograde motion occurs as the faster moving Earth catches up and then passes by. As we observe the outer planet with respect to the stars in the background it appears that the outer planet slows down and then reverses its eastward direction toward the west, and then after a time reverses its direction again, but this time toward its regular eastward direction of motion.

During this month the rapidly moving planet Mercury shakes things up a bit by making its best appearance of the year during May. Observe the skies over west-northwestern horizon shortly after sunset during the first two weeks of the month and Saturn and Jupiter will be visible. Keep an eye on this area as the rapidly moving Mercury passes by Saturn and then Jupiter. This animated image starts on 6 May with Mercury north of Saturn. At one-day intervals Saturn quickly disappears below the horizon while Mercury climbs higher above the horizon toward conjunction with Jupiter on the 15th-16th.