Earth’s Place in the Universe
Day, night, and shadows
Earth spins on its axis once about every 24 hours. The side facing the Sun has day; the side turned away has night. This rotation makes the Sun appear to cross the sky from east to west and changes shadows: shadows are longest in early morning and late afternoon, shortest near noon, and they point away from the Sun. The Sun does not actually travel around Earth; the motion we see is apparent.
Seasons and yearly patterns
Earth also travels around the Sun once a year, but seasons are caused by Earth’s tilted axis, not by our distance from the Sun. When your hemisphere tilts toward the Sun, sunlight strikes more directly and days are longer, so it is summer. Seasonal patterns include changing sunrise and sunset times, changing day length, and different constellations visible at night.
Traps to watch for
- “The Sun is bigger and brighter than other stars.” It is an ordinary-sized star that looks large and bright only because it is very close.
- “A bright star in the sky must be a huge star.” Brightness depends on both true brightness and distance.
- “Seasons happen because Earth gets closer to the Sun.” Tilt explains seasons; both hemispheres have opposite seasons at the same time.
- “The Sun disappears at night.” It is hidden from view by Earth’s rotation, not switched off.
Worked example
A question gives shadow lengths at 8 a.m., noon, and 4 p.m. Rule of thumb: the shorter the shadow, the higher the Sun is in the sky. The shortest shadow is near noon.