Glossary
Find short definitions of words used in the simulations. Search for a term, then open its question for a fuller explanation.
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Asteroid belt
The asteroid belt is a region between Mars and Jupiter containing many small bodies orbiting the Sun, including the dwarf planet Ceres. What is the asteroid belt?
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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 is a blue moon?
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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 daylight saving time?
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Daytime moon
The daytime Moon is the Moon seen during daylight, when it is above the horizon and bright enough to see. Can the moon be up in the daytime?
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Dwarf planet
A dwarf planet orbits the Sun, is nearly round under its own gravity, has not cleared its orbital neighborhood, and is not a moon. Why is Pluto a dwarf planet and not a planet?
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Equinox
An equinox is an instant when the Sun crosses the celestial equator, occurring once in March and once in September. What is an equinox?
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Far side of the Moon
The far side of the Moon is the hemisphere that mostly faces away from Earth; it receives sunlight too. Does the Moon have a dark side?
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How moons form
Moon formation is the process by which natural satellites develop, including formation around a planet, impacts and capture. Why do planets have moons, and how are they made?
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International Date Line
The International Date Line is an imaginary boundary across the Pacific where the calendar date changes by one day. What is the International Date Line?
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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 a leap year?
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Libration
Libration is the Moon’s apparent rocking as seen from Earth, which lets us see a little beyond its usual edges over time. What is libration?
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Mars’s colour
Mars’s reddish color comes mainly from iron oxides in dust on its surface. Why is Mars red?
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Military time
Military time is a way of writing 24-hour time using four digits, such as 1430 for 2:30 PM. How do you calculate military time?
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Moon phase names
Moon phase names describe eight stages of the cycle: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter and waning crescent. What are the moon phases called?
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Moon phases
Moon phases are the changing views of the Moon’s sunlit half as it orbits Earth. Why does the Moon change shape?
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Moonrise delay
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 does moonrise get later each night?
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Orbital motion
Orbital motion is movement around another object under the pull of gravity. How does an orbit work?
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Orbital tilt of the Moon
The Moon’s orbital tilt is the angle of about 5° between its orbit around Earth and Earth’s orbit around the Sun. Why isn’t there an eclipse every month?
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Planet formation
Planet formation is the growth of planets from material in a disk around a young star. How were the planets formed?
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Saturn’s moons
Saturn’s moons are natural objects that orbit Saturn, ranging from large worlds such as Titan to small icy and rocky bodies. Why does Saturn have so many moons?
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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 a solstice?
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Spring and neap tides
Spring tides have a larger tidal range near new and full Moon; neap tides have a smaller range near first and last quarter. What is a spring tide?
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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 supermoon?
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Synodic month
A synodic month is one complete cycle of Moon phases, lasting about 29.5 days. What is a synodic month?
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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 tidal locking?
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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 causes tides?
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Triton’s retrograde orbit
Triton’s retrograde orbit carries it around Neptune in the opposite direction to Neptune’s rotation. Why does Triton orbit Neptune backwards?
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Twilight
Twilight is sunlight scattered through the atmosphere while the Sun is below the horizon. What is twilight?