The moon is waxing now, as it does for about half of its 29-day full orbit around the Earth.
When we have a “new moon,” the moon is between us and the sun and the sun illuminates only the side of the moon we can’t see: all of the side we can see is in shadow. After the new moon, the moon “waxes” as more of the surface is lit by the sun progressively each night we see it during its orbit around us. The sunlit patch of moon grows from the leading or western edge of the moon as it moves around the Earth.
At “full moon” the moon is on the other side of the Earth from the sun and the sun’s light streams past us to illuminate the whole side of the moon that faces Earth. As the moon continues its orbit around Earth, we observe the lit area to become smaller (“waning”), with the shadow expanding from the leading, western edge until the moon is totally “new” again.
We see only one side of the moon though the phases because the moon makes one rotation per orbit around the Earth, always keeping its “face” to us. The moon’s more obvious movement through the sky, like the rise and set of the sun, is due to Earth’s rotation.
Rare among planets, our moon’s volume is about 1/50th the volume of Earth: most moons are much smaller than the planets they orbit.
The current leading theory of moon formation is that just over 4.5 billion years ago another planet — about the size of Mars, called “Theia” — hit our young Earth with a glancing blow. A little of Theia and some of the Earth’s developing crust were knocked into space. Gravity drew the blasted bits together into a sphere and held the sphere in an orbit around the Earth.
While birthing the moon was a tremendous blow, the result has had immeasurable benefits for us.
That giant whack that formed our moon likely also tipped the Earth’s axis a bit; it’s the tipped axis that gives us the seasons. Furthermore, it seems the pull of the nearby moon helps stabilize and maintain that tip.
Some researchers pin the origin of plate tectonics on the catastrophic event that spawned the moon, believing the glancing blow that knocked off pieces left the remaining Earth reeling with currents in the molten rock. Further, that whack on the coalescing planet might have helped mix the forming rocks to give us metals at the planet’s surface, not just at the core.
Our moon has also benefitted the Earth by catching some asteroids or meteors that might have otherwise hit us, making the characteristic lunar craters on the moon’s surface.
We’ve long known that the gravity from our large and near moon pulls on our planet’s seas, creating tides. Besides exposing a bit of ocean-bottom to land animals, moon-driven tides may help stabilize climate by mixing cooler and warmer waters. And some researchers postulate lunar tides mixed the early seas enough to help form organic chemicals that led to life.
Early-Earth tides were larger than today’s tides because the moon was closer to Earth than it is today, affecting a greater gravitational pull. The moon, now 238,863 miles above, is still moving away from Earth at a rate of about 4cm/year.
The next full moon, September 27, will feature a full lunar eclipse as the moon passes through the shadow cast by Earth — a reddish shadow — our next one won’t be until January 2018. The moon will rise above the eastern horizon about 7 p.m., eclipse already underway. Full eclipse will be about 7:45 p.m.; the eclipse will end about 10:20 p.m.
Weather permitting, we’ll see a colorful show from a vital partnership.