The Moon, like the planets and the Sun, travels along a special path in the sky called the ecliptic. Observationally, the path arcs generally across the southern half the sky from west to east, but it is not a simple arc that is in the exact same part of the sky year round. Rather, it curves higher in the sky and lower in the sky as the seasons change.
Right now, in late winter, the Sun remains low in the sky but is gradually climbing the ecliptic, getting slightly higher each day. The Moon this week is just past full, and therefore is traveling along the opposite side of the ecliptic in a part of the sky where the ecliptic follows a low arc in the sky from west to east. And because the Moon takes 29 days to circle the Earth once, and the Sun appears to take 365 days to "circle" the Earth once, we can observe the Moon's motion along the ecliptic much more readily than the Sun's. The image helps to visualize this over the course of four days in which the position of the Moon at the same time in the dawn (an hour before sunrise, about 5:30 to 5:45 am this week in San Francisco) traces out the low, sloping arc of the ecliptic -- and slices close to Saturn as well.
A Great New Image of Saturn
1 week ago