Astro Bob blog: The unstoppable terminatorWhy the moon goes through phases and what the terminator has to do with it. Still time to catch the space station-shuttle duet.
By: Bob King, Duluth News Tribune
The unstoppable terminator
The flag next to the Duluth Public Library flaps in the wind toward the crescent moon earlier this week. The ghostly glow of earthshine is visible above the crescent. Details: 180mm telephoto at f/2.8, ISO 1600 and 1/50" exposure. Photo: Bob King
The lunar crescent was a big banana last night and bright enough to set the landscape softly aglow. Tonight the moon will pull up alongside the Pleiades or Seven Sisters star cluster in Taurus. Matter of fact, the moon, Pleiades, Hyades and Orion all parade together high across the southwestern sky around 8 o'clock.
The moon will be near the Seven Sisters cluster tonight and tomorrow night as it transitions from a thick crescent to half. This map shows the sky as you look toward the southwest around 8 p.m. Maps created with Stellarium
Because the moon is less than half-lit tonight (40 percent), the boundary separating the bright, daylit part from the part still not in sunlight will be slightly curved. This boundary is called the terminator. Sunday night the moon will be seven days old, exactly half-lit and the terminator will run straight up and down. Try to visualize the terminator as the advancing edge of sunrise on the moon's surface. The moon never sits still but is always moving apace in its orbit around Earth. Before full moon, the terminator advances toward the east (left) and reveals a new slice of lunar landscape each night. After full moon, the moon's phase lessens or "wanes" and the terminator becomes the advancing line of lunar sunset. When the sun has set over most of the moon, we see it as a thin crescent in the morning sky at dawn. The moon disappears for a day or two around new moon phase because it's lost in the glare of the daytime sun. When it reappears as an evening crescent, the terminator once again becomes the advancing line of lunar sunrise. The changing phases and shifting terminator of the moon are ultimately caused by the ever-changing angle between Earth, sun and moon during the moon's monthly orbital cycle. Use the illustration below to help guide you through the phases.
A full cycle of lunar phases as seen from the northern hemisphere. Sunlight shines from the right. Starting from left, the moon begins its cycle in new moon phase and waxes toward full. After full moon, it wanes to a thin crescent and returns to new to repeat the cycle. Credit: Wiki commons
I didn't get up this morning to see the shuttle-space station combo but others who did say the sight was very cool. Here's Lyle Anderson's report from Duluth: "I decided to step out and glance at the sky, and the sky was nice and clear. I did not think I would have had time to get the camera and tripod set up, and I did not want to miss the flyover. What a spectacular sight, and there they were cruising across the sky northwest to southeast and so bright. Endeavour leading ISS by a few degrees and getting up to around 68 degrees at the highest. Endeavour was not quite as bright as the station but still quite bright. What a spectacular site cruising across the sky."
We'll have one more opportunity to see the two ships tomorrow morning before the shuttle lands tomorrow night at Kennedy Space Center in Florida.
The International Space Station and shuttle Endeavour made an eye-catching pair this morning in this photo taken by Marco Langbroek of Leiden, the Netherlands. At the time they were separated by just a couple degrees and crossed the sky in tandem. Credit and copyright: Marco Langbroek