Let’s have fun. Here’s how the universe is easy and mind-stretching: Which is the easiest sky-direction to find? What’s the only direction you never have to think about? Answer: the zenith – straight up.
When you look overhead, you peer through the least amount of the air between yourself and outer space. By day, this is the darkest-blue part of the sky. At night, it’s the least-obstructed.
We’re bringing this up now because two cool items hover directly at the zenith – which is actually very unusual. For most people in the US, no bright star ever reaches its zenith. But at our latitude, there are three.
In midwinter, the brilliant star Cappella reaches our zenith. And right now we see the other two. Quite remarkably, all three of these luminaries ascend to within four degrees of the precise overhead point. Since most people cannot distinguish the exact zenith and will consider an object displaced as much as ten degrees from it to be directly overhead, we really score bingos in that department.
Time matters. Things in the sky move so rapidly that in a mere hour, a star at the zenith will have fully shifted 15 degrees away from it. And yes, only stars reach the zenith. From nowhere in the continental US or Canada does the Moon or Sun or any planet ever get straight up.
These nights, the first brilliant overhead star is Vega. It’s directly overhead as darkness falls. Between 7:30 and 8 p.m. it hovers within four degrees of the zenith. You can’t miss it; just crane your neck and that blue overhead star is Vega. Here are three cool facts about it.
First, be sure to say VEE-ga and not VAY-ga. This star was originally WEE-ga, as my late poet friend Janine Pommy Vega liked to point out; the name means a falling eagle or vulture. Our second fact is that Vega, a fairly nearby star at only 25 light-years, is incredibly fast-spinning. Though three times the Sun’s diameter, it whirls around in less than a day, with its equator zooming at 140 miles a second. And guess what? Its pole of rotation points in our direction. We are its North Star. In 12,000 years it will return the favor and become our North Star.
Exactly two hours later – meaning between 9:30 and 10 p.m. – a different star stands straight overhead. It’s not as bright, but just as unmistakable. And interestingly enough, it too hovers within four degrees of the zenith. This is Deneb. It may be my favorite star in all the heavens (although I’ll probably name a different star on another occasion).
Deneb marks the straight-ahead direction as Earth and our Sun whirl around the center of our galaxy. We will arrive at Deneb’s current position one million years from now – although Deneb will no longer be there, since it’s moving forward in the same direction.
Even more remarkably, Deneb is probably the farthest of all the naked-eye stars. No one is completely sure, but it seems to lie 3,000 light years away, and maybe even a bit more than that. For it to appear as bright as it does makes it the most luminous of all naked-eye stars. If it were located where Vega sits, it would shine with nearly the brilliance of the Full Moon.
So during this Full Moon weekend, when only the brightest stars poke through the lunar glare, have some fun looking straight up at nightfall, and maybe again two hours later. If you want to catch both stars at one sitting – well, when Vega is highest, between 7:30 and 8 p.m., you can easily identify Deneb, since it will then be the second-highest bright star.
Cool astronomy made easy.
Want to know more? To read Bob’s previous “Night Sky” columns, visit our Almanac Weekly website at HudsonValleyAlmanacWeekly.com.