Those of you who got up early to see the Perseid meteor shower may have also noticed a new visitor to the dawn sky — Orion the Hunter. Although August seems too early to dip into the winter stars it's one of few times that we can enjoy the constellation in shirtsleeves. For much of the year Orion's all frozen fingertips and frosty breath. At least for northern hemisphere skywatchers. If you live in Rio de Janeiro the Hunter strolls around in shorts with a gin fizz on hot, summer nights.
I like to get up early at least once a summer and make a pilgrimage to Orion. I enjoy seeing the progress of the constellations over the year, and the group is one of my favorites. Orion first appears in the morning sky in early August like fresh bread out of the oven. It's an apt comparison because during the summer months the constellation shares the sky with the blazing sun, the reason we can't see it at that time.
Earth turns on its axis and also circles around the sun. As kids we learned Earth's spin causes the stars to rise, travel west across the sky and then set. You can see this for yourself any clear night. Just eyeball a star low in the eastern sky, return a a half-hour later, and sure enough, it will have risen higher.
Our planet's orbital motion nudges stars and planets from east to west too, but more slowly. As Earth zips along its orbit at 1.6 million miles (2.6 million km) per day it causes the stars and planets in the eastern sky to rise 4 minutes earlier each night and those in the west to set 4 minutes earlier. Over time the minutes add up, causing the constellations to slide from east to west over the seasons.
It's amazing how quickly this happens. After just a single month, stars in the east rise2 hours earlier.To make room for all those "new" constellations pushing up from the eastern horizon, those in the west must go bye-bye. No problem. They set 4 minutes earlier each night, making way for their usurpers.
Orion's appears in the east at dawn in mid-August, reaches the same position at midnight in mid-October and shines high in the southern sky at the same time in mid-December. Come mid-winter, it's due south at nightfall and then tilts westward in spring until it disappears below the horizon in May. From late May through July Orion's still there but strides invisibly across the daytime sky invisible in the solar glare until it returns at dawn in August.
Groups like Orion, Taurus and Gemini are called winter constellations because that's when they're easiest to see during the evening hours. Leo and Virgo belong to spring, Sagittarius to summer and Pegasus to fall. It's all just a big, merry-go-round. Now that you know Orion's been there all along, set your alarm for 4 a.m. (local time) and watch those magical three stars twinkle in the eastern sky before the sun comes up. Careful though — don't get bit by a mosquito.
Tomorrow, we'll look at the latest results from the Hubble Space Telescope that may explain why Orion's most famous star — Betelgeuse — grew so faint last winter.