Ricochet is the best place on the internet to discuss the issues of the day, either through commenting on posts or writing your own for our active and dynamic community in a fully moderated environment. In addition, the Ricochet Audio Network offers over 50 original podcasts with new episodes released every day.
NASA’s Deep Space Climate Observatory (DSCOVR) has captured a series of unusual images showing the astronomical phenomenon known as the transit of the Moon across the Earth. They are unusual because out of the handful spacecraft are beyond the orbit of the Moon, very few are close enough to perceive the Earth and Moon as larger than a speck. The video below is not a computer simulation; it is a series of actual photographs taken on July 16, 2015.
As the Moon passes in front of the Earth, the visible portion of the Moon is what we Earthlings refer to as the far side, not the “dark side” as reported in many news outlets. Since the Moon is tidally locked with the Earth — meaning it rotates at exactly the same angular rate that it orbits — it always presents us the same face. In other words, the Moon has a permanent far side that cannot be seen from Earth. The far side has an entire set of craters and other surface features that were completely unfamiliar to us until the Space Age.
But, as any child can observe, the portion of the Moon that is illuminated by the sun goes through phases, meaning that different parts of the Moon are illuminated at different times. So the “dark side” is not a fixed feature but rather a cyclical phenomenon exactly like night and day on Earth. The far side of the Moon sees just as much sunlight as the near side, only on the opposite schedule.
DSCOVR is positioned at a unique point in space called the Sun-Earth L1 Lagrange Point, or L1 for short. In general, any system of orbiting bodies has a set of Lagrange points, and a two-body system has five such points. Think of our Lagrange points as the balancing points between the Sun’s and Earth’s gravitational pulls and the orbital centrifugal force. An object placed there will not fall toward the Earth or Sun, nor will it pass ahead or behind. Its orbital angular speed will match the Earth’s, and it will seem to be suspended in place relative to the sun and Earth.
The L1 point is the equilibrium point that lies about one million miles on a direct line from the Earth toward the Sun. From the Earth’s perspective, an object at L1 always remains directly in front of the sun. If the Moon passes between the Earth and L1, the far side of the Moon is visible from the L1-orbiting spacecraft, fully and brightly illuminated by the sun. It’s disheartening the way so many news outlets are reporting this exactly wrong. Viewed from L1 the Moon is full, so the only part of the Moon not visible in the images is the dark side, which, incidentally, is coterminous the Moon’s near side at that time.
Now, since you are a Ricochet member — and I know you love to do so — you can go around correcting people for the rest of the day.Published in