To continue our astronomy lesson from last month...

Planet Earth rotates on its axis at 24-hour intervals, which we call a "day," and it takes 365 1/4 days to revolve around the sun, which we call a "year." (More correctly, an "Earth day" and "Earth year.") The moon revolves around planet Earth in 29 1/2 days (a "month," meaning "moon"), changing phases from "new" (when its aligned with the sun) to "full" (when its opposite the sun).

The eight visible planets follow the line of the "ecliptic" (the imaginary line through the sky that the sun and moon also follow), and their progress in the evening sky depends basically on the length of their orbit around the sun. One classification of the planets is that they're "exterior," beyond Earth's orbit, and "inferior," between Earth and the sun.

Only two planets are closer to the sun than Earth: Venus and Mercury. And because of their orbit, they never stray far from the sun. Venus is currently not visible, but the first week of April, the elusive Mercury is an evening "star," low in the west after sunset.

The other evening "star" this month is Jupiter (an exterior planet), in Leo, and out all night. Mars and Saturn (both in Ophiuchus, the "serpent bearer"), are our sole morning "stars."

This lesson will conclude next month and I'll see you then!