Questions about time, Earth and space
Choose a question about the Moon, Earth, planets or time. Read a short answer, then use a related simulation to explore it.
Browse by phenomenon
Moon, tides and eclipses
Change the Moon’s position, spin, distance and orbital tilt. Then connect what moves to phases, moonrise, eclipses and tides.
What is a synodic month? A synodic month is one complete cycle of Moon phases, lasting about 29.5 days.
What is tidal locking? Tidal locking is rotation that takes the same time as one orbit, keeping the same side facing the object being orbited.
What is libration? Libration is the Moon’s apparent rocking as seen from Earth, which lets us see a little beyond its usual edges over time.
Why does moonrise get later each night? Moonrise delay is the change in the Moon’s rising time from one day to the next; it usually rises later as it moves along its orbit.
Why isn’t there an eclipse every month? The Moon’s orbital tilt is the angle of about 5° between its orbit around Earth and Earth’s orbit around the Sun.
What is a supermoon? A supermoon is a new or full Moon near its closest point to Earth; the term is most often used for a full Moon.
What is a blue moon? A blue moon commonly means the second full Moon in a calendar month; another definition is the third full Moon in a season containing four.
What are the moon phases called? Moon phase names describe eight stages of the cycle: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter and waning crescent.
Does the Moon have a dark side? The far side of the Moon is the hemisphere that mostly faces away from Earth; it receives sunlight too.
Light, day and seasons
Explore twilight, solstices, equinoxes and the daytime Moon.
What is twilight? Twilight is sunlight scattered through the atmosphere while the Sun is below the horizon.
Can the moon be up in the daytime? The daytime Moon is the Moon seen during daylight, when it is above the horizon and bright enough to see.
What is a solstice? A solstice is an instant when the Sun reaches its farthest position north or south of the celestial equator during the year.
What is an equinox? An equinox is an instant when the Sun crosses the celestial equator, occurring once in March and once in September.
Gravity, motion and the universe
Explore how orbits work and how the Moon affects tides.
How does an orbit work? Orbital motion is movement around another object under the pull of gravity.
Why does the Moon change shape? Moon phases are the changing views of the Moon’s sunlit half as it orbits Earth.
What causes tides? Tides are changes in sea level driven mainly by the Moon’s and Sun’s gravity, with local patterns shaped by coastlines and the ocean floor.
What is a spring tide? Spring tides have a larger tidal range near new and full Moon; neap tides have a smaller range near first and last quarter.
Time from the sky and clocks
Explore daylight saving time, leap years and the International Date Line.
What is daylight saving time? Daylight saving time is a seasonal change to local clocks, usually one hour ahead of standard time during part of the year.
What is a leap year? A leap year has an extra day to help keep the calendar aligned with Earth’s orbit; in the Gregorian calendar, that day is February 29.
What is the International Date Line? The International Date Line is an imaginary boundary across the Pacific where the calendar date changes by one day.
Worlds, materials and formation
Compare planets, moons, atmospheres and the asteroid belt, then ask what their differences say about how the solar system formed.
