Astro Bob blog: Lunatics part II plus things to see while waiting for the fireworks to beginOur final night on the moon reveals how slowly the stars there appear to move. Watching the fireworks? Show your friends summer's brightest stars while you wait for the show to begin.
By: Bob King, Duluth News Tribune
Lunatics part II plus things to see while waiting for the fireworks
The sky seen from mid-northern latitudes on the moon tonight. Earth would be near Virgo's brightest star Spica. Scorpius the Scorpion is low in the southeast. See below for the sky four days later. Maps created with Stellarium
Yesterday we went to the moon and learned that the Earth pretty much stays in one spot in the sky that depends upon where you're standing. Near the center of the lunar disk, the Earth is overhead; in mid-northern latitudes it's halfway up in the southern sky, and if you're anywhere along the outer perimeter of the moon's face or near the poles, it's close to the horizon. This seemingly odd circumstance arises because the moon's rotation is in synchrony with its revolution around our planet, so that only one side of the moon always faces the Earth. Lunar sky watchers always stare earthward the same way earthlings only ever see the lunar nearside.
One evidence of a rotating Earth is the rising and setting of the stars. At the onset of darkness in July, we see Leo the Lion in the western sky. Two hours later, it's set and new stars have risen in the east. 24 hours later we see Leo again. This goes on day and night and reflects the fact of our planet's 24-hour west to east rotation.
A lunar observer always faces the Earth as the moon rotates on its axis, but the sky background (constellations) changes during the moon's nearly month long day-night cycle. Our observer would see the constellations move across the sky -- albeit more slowly -- during the time it takes for the moon to revolve around the Earth. Illustration: Bob King
The moon rotates once every 27 days making the lunar day 27 times longer than Earth's. Any particular place on the moon experiences about two weeks of night and two weeks of daylight. Now imagine a little skywatching during the bitter cold lunar night. The constellations would slowly drift to the west during the two-week-long evening at a rate 27 times slower than on Earth. Leo would remain visible for nearly two days in the western sky before setting. To see Jupiter, which rises after local midnight, would mean waiting more than three days after lunar sunset for it to finally appear above the eastern horizon. I'd probably sneak a little nap in there somewhere.
Four Earth-nights later on July 8, Scorpius is up in the south and Earth has a striking conjunction with Antares. Notice also that our planet has filled out to nearly full phase. Over a lunar day-night cycle, the Earth stays in approximately the same place in the sky. The moon's slightly inclined orbit causes the Earth to appear to move up and down a little, while changes in the moon's orbital speed likewise cause the Earth to shimmy back and forth by a similar amount.
So now you can picture a very slow parade of constellations creeping across the lunar sky day and night with the Earth hovering in the foreground. Strange, isn't it? And we didn't have to travel too far from home to find such curious circumstances. Given all the planets and moons with their varying orbits and rotation periods, there are so many different and interesting ways of envisioning the night sky across the solar system.
Plan on watching the fireworks tonight? Let's hope it's clear. If so, you can look for the brightest stars and planets come out while you wait for the show to begin. The International Space Station will also pass over many locations around 10 o'clock. For Duluth, the ISS glides below the Big Dipper in the northern sky beginning at 9:52 p.m. A later pass at 11:28 p.m. will take the station across the top of the sky. For times for your town, click HERE and type in your zip code.
This map shows a nearly 360-view of the sky this evening around 10-10:30 p.m. local time. Watch for Venus to show first in twilight. "S" stands for Saturn and "M" for Mars.
The easiest "star" to see will be brilliant Venus, low in the northwestern sky, while the first true star to pop out will likely be Arcturus, high in the southwest. If you turn around and look high in the eastern sky, Vega should be easy, too. Altair and Deneb will require more attention and a little more darkness as will Spica and the planets. And don't forget Antares in Scorpius low in the southeast. When all the sparks and explosions fade, these little points of light will remain.