We had a great few weeks turning our eyes from the night sky to the daytime sky and things you can see here on Earth, but this week we’ll be looking back at the night sky.
We’ve had a lot of clouds over the past month or so as the winter stars have been rising. Orion is now located high in the sky after sunset making for excellent viewing of the many nebulae located near it’s belt. Jupiter is growing lower in the evening sky but still shines brightly. Saturn has become invisible as it begins the transition from morning to evening star.
Over the past month, Venus has shifted from the evening sky to the morning sky. During this transition it has progressed through its crescent phase and has now been getting a little wider when viewed through a telescope. Over the coming months it will begin a grand tour of its fellow planets while remaining fairly low in the morning sky. Right now it is located near Mars but it will begin a slow crawl towards the red planet in the morning sky. This crawl will last this month with the closest approach coming in early March.
Venus also appears as a crescent right now, only about 25% illuminated. When viewed through a telescope it appears as a tiny crescent moon. This doesn’t stop it from being very bright, though. It’s proximity to the Earth will only be about 38 million miles, around 40% of the distance from the Earth to the Sun. This closeness will allow Venus to be magnitude -4.86 (VERY bright). This is nearly the maximum brightness Venus can possibly attain.
Another interesting phenomenon to take a peek at this month (and every month) is called lunar libration. Everyone knows the moon has a “far” side and a “near” side, with the near side being the only side ever visible from Earth. This doesn’t mean that we only ever see 50% of the Moon’s surface, however. As the moon orbits the Earth, it rotates on it’s own axis. Its day and its “year” (or month in the moon’s case) are the same length. This is known as being tidally locked and is the case with every large moon in the solar system. The moon became tidally locked because, over time, the Earth’s gravity slowed down the moons rotation to match its orbit length.
Even though the moon is tidally locked, it does still shift slightly from our perspective. Over time, around 59% of the moon’s surface can be visible from Earth. As the moon orbits the Earth it doesn’t go in quite a perfect circle, and it also moves quicker and slower. These changes cause it to wobble a bit both up/down (north/south) and side to side (east/west). This allows different geographic features along the moon’s edge to appear and disappear. The GIF below shows the moon’s appearance during one full orbit. Notice it grows larger and smaller as it moves along its elliptical orbit.
The different areas visible vary a bit from month to month as our perspective of the moon’s orbit changes. Below is an image of the lunar surface. The areas in yellow are what would be visible without this libration, and the area inside the green is what we are able to see thanks to it.
So, over the coming months be sure to take a peek at the moon’s edges. You just might see a crater you’ve never seen before!
Everyone have a great week and, as always, watch the skies!
