Why does the Moon have phases?
We see it almost every night and puzzled on why its there and has phases. The Moon has phases because it orbits Earth, which causes the portion we see illuminated to change.
The Moon takes 27.3 days to orbit Earth, but the lunar phase cycle (from new Moon to new Moon) is 29.5 days. The Moon spends the extra 2.2 days “catching up” because Earth travels about 45 million miles around the Sun during the time the Moon completes one orbit around Earth.
At the new Moon phase, the Moon is so close to the Sun in the sky that none of the side facing Earth is illuminated. In other words, the Moon is between Earth and Sun. At first quarter, the half-lit Moon is highest in the sky at sunset, then sets about six hours later. At full Moon, the Moon is behind Earth in space with respect to the Sun. As the Sun sets, the Moon rises with the side that faces Earth fully exposed to sunlight.
You can create a mock-up of the relationship between Sun, Earth, and Moon using a bright lamp, a basketball, and a baseball. Mark a spot on the basketball, which represents you as an observer on Earth, then play with various alignments of Earth and Moon in the light of your imaginary Sun.
Why does the Moon look bigger at the horizon?
The “Moon illusion,” in which the Moon appears larger than normal when close to the horizon, is not the result of magnification by the atmosphere or a change in Earth-Moon distance. Instead, the answer is, as Einstein might say, completely relative.
At most times we see the Moon high in the sky among thousands of stars. We develop our sense of how “big” the Moon ordinarily appears by comparing it with the vast panorama of outer space.
When the Moon is nestled along the horizon, however, we see it surrounded by a foreground of familiar Earth-bound objects — trees, buildings, or distant landmarks. In comparison with these everyday features, the bright disk of the full Moon appears quite large indeed, and relative to our “normal” sense of the Moon’s size, much bigger than we would expect.
Why do stars twinkle?
Because stars are so incredibly distant, to our eyes they appear strictly as points in the night sky. Irregularities in Earth’s atmosphere cause starlight to dance around, and the minute changes in the path the starlight takes through the atmosphere results in apparent changes in colour – the familiar “twinkling” effect.
Planets, however, actually form a tiny but definite circle on the sky just large enough to counter the distorting effect of turbulence. Such extended objects only “twinkle” when their light passes through very large amounts of atmosphere, such as when they lie close to the horizon.