Our moon's form does not actually change; it simply appears to! The "amount" of the moon that we see when we gaze at it from Earth changes in a monthly cycle (29.5 days). These variations are caused by the relative locations of our Sun, Earth, and Moon. When the Moon is on the other side of Earth where it is night, we can't see it. But when it comes into view again during the day, it reveals its presence by dimming or eclipsing the sun.
The eclipse of July 2-3, 2019 will be a total lunar eclipse. It will be the first total lunar eclipse of the 21st century and the fourth of five total lunar eclipses in 2019. At maximum darkness, when only the brightest stars can be seen with the unaided eye, the eclipse will be visible from everywhere on Earth except North America.
Lunar eclipses are visible on half of Earth because the Moon is covered by the shadow of Earth. As this dark area moves across Earth, so too does the region where people can see the eclipse. Because all direct sunlight is blocked out, only light from the Sun as refracted through Earth's atmosphere reaches it. This has two important results: first, because all its direct light is obscured, the Moon will appear dimmer from Earth than otherwise; second, a certain part of Earth's surface - that facing towards the Moon - must at least partly be sunlit during an eclipse.
The Moon's fluctuating locations as it revolves our planet cause the Sun to illuminate different sections, generating the appearance that the Moon's form is changing over time. This is due to the fact that it spins once on its axis in precisely the same amount of time as it takes to circle Earth—27 days and seven hours. The further away from the equator it is, the faster it rotates.
The Moon is always facing us out into space, but as it orbits Earth it passes between the Sun and ourselves at various distances. At perigee (closest approach) it goes completely around Earth, while at apogee (farthest distance) only a small part of it is exposed. Because light travels more slowly when it has to travel through air or water, at perigee more of the Moon is illuminated than at apogee.
This is why when the Moon is at its brightest, it is not always directly ahead of you. Sometimes it can be to the east, west, north, or south of where you think it should be. As we watch it move across the sky, it does so because we are all in motion around Earth's axis, and thus the Moon is affected by our location relative to Earth's rotation. If we were standing still with respect to Earth, the Moon would appear to stay in the same place in the sky throughout its monthly cycle.
As a result of the Earth and Moon orbiting the Sun and the Earth spinning once every 24 hours, the Moon's appearance and location vary. As the Moon orbits the Earth, it goes through phases: full, half, and crescent. As well, over time the shape of its orbit will slowly change due to gravitational forces from other bodies.
The last major change to occur in the Earth-Moon system was when the moon passed through the debris disk left by the impact that destroyed the planet Venus billions of years ago. This event altered the orbit of the moon and pushed it away from Earth toward Mars. The distance between Earth and Moon has since decreased due to other events happening within the solar system, but because of this collision they are now at their closest point ever - about 423,000 miles (700,000 km) apart.
During a lunar eclipse, the Earth's atmosphere refracts light from the Sun that reaches and enters it from behind the Moon. This only happens during certain parts of the month as the Moon's orbit brings it across different positions in relation to the Earth. As we watch these events unfold we are seeing a small part of what is actually happening on the Moon!
Because we view various portions of the lighted part of the Moon as it revolves Earth, it appears to change form. The lighted side of the moon is concealed from us when it lies between Earth and the Sun. As it travels around the Earth, more and more of the illuminated side becomes visible. Then it starts to go away again. When it has gone all the way around to the far side, no new parts are exposed to view.
This is why there is a full moon on some nights and only the crescent moon during another phase of the moon. As well as being visible at night, the moon also causes tides to rise and fall. These tides occur because water soaks up more or less space depending on whether there is land or not near where it is located. This means that high tide occurs about an hour after sunset and low tide is about an hour before sunrise. Tides are highest in the morning when the sun is rising over land and lowest in the evening when land is hidden by darkness.
The cycle then begins again with the next new moon.
As well as changing shape, the moon is also affected by other bodies in our solar system. If the Earth was alone in orbit around the Sun, then the moon would always be fully illuminated even if it was directly facing out towards space. But because we live on a planet, the moon is always obscured to some degree by Earth's atmosphere.