Explore in 3D · Sky model

Sun, Earth and Moon

Pick any day. See where Earth and the Moon are, what shape the Moon is, and how long the daylight lasts.

Flat view: Tilted Earth

Goes with

The sky model

A model, not to scale

The Sun lights half of Earth and half of the Moon. Press a picture button, or pick a day.

Try this

  1. Press Play a month. Which side of the Moon is lit? Look for the Sun.

  2. Put your birthday in the date box. Press From Earth. What shape was the Moon?

  3. Press True scale. How many Earths fit between Earth and the Moon?

  4. Pick Anchorage, then Quito. Press Play a year. Which place has long summer days?

What did you notice?

  • The lit side of the Moon always faces the …
  • Does Earth's tilted axis keep pointing the same way all year?
  • Is the Moon close to Earth, or far away?
  • Half of the Moon is always lit. Why do we see different shapes?

Tell a grown-up, or draw it in your workbook.

For grown-ups: what this model shows, and what it changes
  • Real dates. Pick any day from 1900 to 2100. Positions come from approximate formulas: the Sun from the U.S. Naval Observatory's approximate solar coordinates, the Moon from a standard low-precision lunar theory that includes its main wobbles, and Earth's turning from sidereal time. Phase times are usually within about an hour of published tables. This is a model, not an observing calendar.
  • Not to scale, then true scale. "From space" squeezes the distances so all three fit. Earth and Moon always keep their true size ratio (the Moon is about a quarter as wide as Earth). "True scale" shows Earth and Moon at their real sizes and real distance, about 30 Earth-widths apart. The Sun is not drawn to scale: it is about 109 times wider than Earth and about 390 times farther than the Moon.
  • Moon phases. One light, from the Sun's real direction, lights both Earth and the Moon. The Sun always lights half of the Moon. As the Moon goes around Earth, we see more or less of that lit half. Phases are not Earth's shadow; Earth's shadow on the Moon is a lunar eclipse. One cycle takes about 29½ days.
  • From Earth. The Moon pictures use the view from the northern half of Earth, with north up: a growing Moon is lit on the right. From Sydney or other southern places, the Moon looks upside down, so it is lit on the left.
  • Seasons. Earth's axis is tilted about 23.4° and keeps pointing the same way in space. Through the year, each half of Earth leans toward the Sun and then away. That changes daylight hours and how high the Sun climbs. Distance from the Sun is not the cause.
  • Daylight hours. The chart uses NOAA's solar-position equations for each day (sunrise to sunset, including the usual allowance for the bending of light by air), so its numbers are "about". Hills, buildings and weather change real sunrise and sunset a little.
  • Near Earth. This view rides along with Earth. The line around Earth is the equator, and it tilts with the axis. The side facing the Sun has day; the other side has night. The thin orange band is dawn and dusk. The yellow pin is the place you picked. Drag Time of day to turn Earth and watch the pin move into or out of sunlight; drag Day of the year to watch the seasons change.
  • City lights. The night-side lights are drawn by this page from a list of about 240 large cities, so they show roughly where many people live. They are not a photo of Earth at night.
  • Pictures. Earth: NASA Blue Marble. Moon: NASA Lunar Reconnaissance Orbiter color map. Both are public domain.
  • Real observation first. Weeks 1, 23, 24 and 33 ask your child to watch the real sky and keep a record. Use the model afterward to explain the pattern.