This model may be used to demonstrate some of the connections between the Earth, Sun, and Moon. How long would you have to wait to see another full moon if you saw one today?
The answer is about 29 days. The reason for this is that the average distance between the Moon and Earth is about 149 million miles (243 million km). Every month or so, the Moon passes through each of the phases of the Moon: New, First Quarter, Full, Third Quarter. During a given lunar cycle, the Moon will pass over every point on Earth once, but not necessarily in the same order until later cycles when it returns to the same place in the sky.
Since the average speed of the Moon across the face of the Earth is about 30 miles per second (48 kilometers per second), it will take about 29 days to traverse its orbit. For example, if you watched the Moon tonight, it would be getting closer around midnight tomorrow, and farther away after midnight tomorrow night.
The Moon is tidally locked to Earth, meaning that the same side of the Moon is always facing Earth. This is because the pull of Earth's gravity keeps compressing the lunar crust from both sides, creating a high pressure region called a "highland" on either side of the Moon.
We know the Moon is spherical like a ball and that the Sun may shine on all sides of the Moon. The light shines brightly on the face we always see with the full moon. At a new moon, the moon is on the opposite side of its orbit as it was when it was full. So there's no way for the Sun to shine on both sides at once.
However, because of lunar eclipses, where the Earth between Moon and Sun blocks out part of the Sun's light, we can see that part of the Moon during a total lunar eclipse. This means that somewhere on the dark side of the Moon, there is sunlight shining on it right now!
Lunar eclipses happen when the Moon is completely covered by one of Earth's shadows. Because lunar orbits are almost circular, this happens only around half of Earth's surface at any given time. On average, about three quarters of Earth is exposed to sunlight during a lunar eclipse.
Where it is night on Earth, darkness falls on the Moon too. When the Moon is fully covered by Earth's shadow, no direct sunlight reaches it. But when just a thin crescent is visible over Earth's horizon, the far side of the Moon is illuminated by reflected sunlight.
During a total lunar eclipse, the Earth's atmosphere refracts light from the Sun that reaches it to create a reddish hue on Earth's moon.
The Earth moves or circles around the Sun as it spins. The Moon circles the Earth in the same way as the Earth orbits the Sun. The Moon's orbit lasts 27 1/2 days, but because the Earth moves, it takes two extra days, or 29 1/2, to return to the same location in our sky. This is called a lunar month.
When viewed from the Earth, the Moon looks as if it is standing still while the Sun appears to move across it. But this is an illusion caused by the fact that the Moon is actually moving around the Earth every day. If we could see beyond the Earth's atmosphere, we would see that the Moon is in fact spinning on its axis at about 1,000 miles per hour, just like other planets.
The Moon is also rotating around the Earth in the opposite direction, which means that at any given time, one side of the Moon is facing towards us while the other is turned away. This is why there is a difference between "lit" and "dark" sides of the Moon. Lit areas receive sunlight during a part of their rotation, while dark areas never see the Sun. Over time, all parts of the Moon become equally illuminated due to a process called "equal illumination."
Finally, the Earth rotates on its axis once every 24 hours and 50 minutes, and this causes the appearance of daily cycles in nature. For example, plants grow during the daytime and sleep at night.
The new moon phase As the orbit progresses, we notice that more than half of the circle is lighted and that the phase is waxing gibbous. The full moon phase occurs when the moon is completely opposite the sun in the sky and we view the entire lit area of the moon (a full circle). This happens about two weeks after the start of the lunar cycle.
The term "new" and "full" refer to how much of the moon is illuminated at any given time. At first glance, it might appear that only part of the moon is dark. However, if you look closer you will see many small craters across the face of the moon. These pits are usually not more than 10 feet (3 m) deep and cover most of the surface. It is these holes in the moon's crust that cause shadows when the Earth passes between them and the Sun.
These events happen about every 29 days, but because of the moon's own orbit around the earth, this interval varies between 27 and 31 days. Thus, the moon's phase goes through five stages: New, First Quarter, Third Quarter, Full, and Last Quarter.
During a new moon, the center of mass for the moon's orbit around the Earth is on the same side as the planet Earth. So, the moon doesn't pass over any land areas and no night follows day.