At sunrise or sunset you can see an interesting optical effect caused by light’s tendency to travel in straight lines. If you live in a mountainous area, as I do, the effect is even more pronounced. Every year near the Equinox, the Sun sets in a valley west of here. Its light illuminates New River Mesa and some of the other foothills, but is blocked from illuminating the New River Mountains (in the distance) and my locale (foreground). It’s a beautiful interplay of light and shadow.
The inset shows a similar effect, but on the Moon near a major crater named Copernicus. If you watch these mountainous areas through a telescope, you’ll see peaks on the night side slowly become illuminated during the lunar sunrise. What starts out as a tiny point of light (the very peak of a mountain) becomes a fully lit mountainside in a couple hours. You can actually see the illuminated area grow as you watch.
But there’s one big difference. On Earth, once the Sun reaches the horizon, and depending on your latitude and season, it can take anywhere from 2 minutes (at the Equator) to weeks (at the poles) for the whole Sun to become visible. For most sunrises in temperate latitudes, the average is 5-15 minutes.
On the Moon, where night and day last for 2 weeks, sunrise is a more gradual process. About the fastest it can happen is one hour. Again, that’s from when the Sun first touches the horizon until it’s fully visible. Check out the time-lapse video below to watch light & shadow on the Moon. The video compresses 4 hours of real-time into a mere 15 seconds.
http://www.youtube.com/watch?v=ZAw7_jq2HPw
By the way, the “X” shaped object you watched coming into view is known as “The Werner X,” after nearby Werner Crater. It’s really just some cliffs and mountains, but shows up as a distinct “X” at lunar sunrise.
Sunrise on the Moon was first witnessed by Galileo in 1610, not long after he invented the astronomical telescope. In his classic work, Sidereus Nuncius (Starry Messenger), he actually used the phrase “sunrise in the mountains of the Moon.” His observations established, as scientific fact, that the Moon has geology and topography much like the Earth, a matter that had been debated for centuries.
I should probably mention one other difference between sunrise (or sunset) on the Earth and Moon. Because Earth has an atmosphere, we experience periods of gradually increasing or decreasing light known as dawn and dusk. Not so on the Moon, which lacks an atmosphere to scatter and diffuse the Sun’s light. On the Moon, when the Sun rises or sets, it’s like throwing a light switch … the transition between day and night is nearly instantaneous.