This evening look toward the western horizon for the brightly shining planet Venus a couple of degrees from the less bright star Spica. Spica in many mythologies represents a bundle of wheat held in the left hand of the Harvest Maiden. Although at this time of the year it looks more the Maiden is doing a back dive toward the horizon!
What does the latter statement mean? Well, try the following observation and you will see the effect of the Earth orbiting (revolving) around the Sun. So from our perspective on the Earth it looks like the the Sun is moving eastward each day. Another way to think about this is to note that each day a star, Spica for example, sets a little earlier, and conversely Spica would rise a little later each day. Maybe not noticeable right away but after a week or so or longer it will.
Confusion clarification time! The above involves apparent motions created by Earth revolution – not to be confused with the apparent motions created by Earth rotation. When facing south in the northern hemisphere, or facing north in the southern hemisphere as the Earth rotates from west to east the ‘sky’ (Sun, stars, planets, Moon) appears to rise above the horizon in the east, moves in a curved path toward the west, and then setting below the western horizon.
Observation: If possible go out at sunset for a few days in a row and make a note of the position of Venus and Spica. After a few days of observing at the same time, at sunset, you will notice that Spica sets earlier each day – is lower above the western horizon at sunset – eventually is no longer visible. Venus will do the same thing but because unlike Spica Venus’s orbit is around the Sun so it will take a few months before Venus is setting earlier and too low at sunset to be visible.
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.