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Sun, Earth, Moon
Sun Earth Moon apparent motion
| Term | Definition |
|---|---|
| Apparent Movement | Motion an object seems to have from the observer's viewpoint; it may result from the observer's motion. |
| Rotation | An object's spin on its axis; Earth's rotation produces the daily cycle of day and night. |
| Revolution | An object's motion around another object; Earth's revolution around the Sun takes about one year. |
| Axis of Rotation | The imaginary line through an object about which it spins. |
| Sunrise | When the Sun appears above the horizon; it generally rises in the eastern part of the sky. |
| Sunset | When the Sun disappears below the horizon; it generally sets in the western part of the sky. |
| Constellations | Named regions of the sky associated with recognizable star patterns; the stars need not be close together in space. |
| Planetary Movement | The changing position of a planet relative to background stars as Earth and the planet orbit the Sun. |
| Retrograde motion | An apparent temporary reversal of a planet's motion among the background stars, caused by changing orbital viewing geometry. |
| Astronomical Unit (AU) | A distance approximately equal to the average Earth-Sun distance: about 150 million km or 93 million miles. |
| Light-year | The distance light travels in one year: about 9.46 trillion km or 5.88 trillion miles; it is not a unit of time. |
| Light-minute | The distance light travels in one minute, about 18 million km. |
| Light-second | The distance light travels in one second, about 300,000 km. |
| Ecliptic | The Sun's apparent annual path among the stars; it corresponds to Earth's orbital plane projected onto the sky. |
| Analemma | The figure-eight pattern traced by the Sun at the same clock time and location through a year, due to axial tilt and Earth's noncircular orbit. |
| Daily apparent sky motion | Earth rotates west to east, making the Sun and most stars appear to move east to west. |
| Why nighttime constellations change with the seasons | As Earth revolves around the Sun, its night side faces different directions in space. |
| Choosing AU or light-years | Use AU for typical solar-system distances and light-years for distances between stars. |
| Light travel time formula | Time = distance divided by speed; keep the distance and speed units consistent. |
| Sunlight travel time to Earth | About 8 minutes 20 seconds; use about 8 minutes if a problem tells you to round. |
| Seasonal sunrise and sunset positions | The sunrise and sunset points shift along the horizon through the year because Earth's axis is tilted. |
| Earth-Moon System | Earth and the Moon interacting through gravity and orbiting their shared center of mass, which lies inside Earth. |
| Gravity Attraction between objects with mass | it depends on their masses and separation. |
| Barycenter | The shared center of mass around which two interacting bodies, such as Earth and the Moon, orbit. |
| Tidally Locked | Having a rotation period that matches the orbital period, so nearly the same side faces the object being orbited. |
| Synchronous rotation | One rotation per orbit; the Moon rotates once on its axis each time it orbits Earth. |
| Synchronous Orbit | An orbit whose period matches a specified rotation period; for this unit's Moon model, focus on the Moon's matching spin and orbital periods (synchronous rotation). |
| Resonance | relationship in which repeating motions have periods in a simple whole-number ratio; the Moon's spin-orbit resonance is 1:1. |
| Lunar phase | The shape of the Moon's sunlit portion visible from Earth. |
| Crescent | A phase with less than half, but more than none, of the visible lunar disk illuminated. |
| Gibbous | A phase with more than half, but less than all, of the visible lunar disk illuminated. |
| Waxing | The visible illuminated portion of the Moon is increasing. |
| Waning | The visible illuminated portion of the Moon is decreasing. |
| New Moon | The Moon is on Earth's Sun-facing side; its illuminated half faces mostly away from us. |
| Waxing crescent | The visible illuminated portion is increasing but is still less than half of the disk. |
| First quarter | About half the visible disk is illuminated and increasing; about one quarter through the cycle from new moon. |
| Waxing gibbous | More than half the visible disk is illuminated, and the illuminated portion is increasing. |
| Full Moon | The Moon is opposite the Sun in our sky, and nearly all its visible disk is illuminated. |
| Waning gibbous | More than half the visible disk is illuminated, and the illuminated portion is decreasing. |
| Third quarter (last quarter) | About half the visible disk is illuminated and decreasing; about three quarters through the cycle from new moon. |
| Waning crescent | Less than half the visible disk is illuminated, and the illuminated portion is decreasing. |
| Eight phases in order | New, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent, then new again. |
| Cause of Moon phases | As the Moon orbits Earth, we see different amounts of its sunlit half; ordinary phases are not caused by Earth's shadow. |
| Phase cycle length | About 29.5 days from one new moon to the next. |
| Moon's orbital and rotation periods | Both are about 27.3 days relative to the stars; the phase cycle is longer because Earth also moves around the Sun. |
| New moon approximate rise and set | Rises near sunrise and sets near sunset; usually difficult to see because it is close to the Sun in the sky. |
| First quarter: approximate rise and set | Rises near noon and sets near midnight. |
| Full moon: approximate rise and set | Rises near sunset and sets near sunrise. |
| Third quarter: approximate rise and set | Rises near midnight and sets near noon. |
| Perigee | The closest point to Earth in an object's orbit around Earth; the Moon is nearer at perigee. |
| Apogee | The farthest point from Earth in an object's orbit around Earth; the Moon is farther at apogee. |
| Solar Eclipse | An event in which the Moon blocks some or all of the Sun from an observer's view on Earth. |
| Lunar Eclipse | An event in which the Moon passes through Earth's shadow. |
| Umbra | The darkest shadow region, where direct light from the source is completely blocked. |
| Penumbra | A partly shaded region, where only some direct light from the source is blocked. |
| Total Eclipse | An eclipse in which the relevant disk is completely covered; for a total lunar eclipse, the Moon is entirely within Earth's umbra. |
| Partial Eclipse | An eclipse in which only part of the Sun's visible disk is covered, or only part of the Moon enters Earth's umbra. |
| Annular Eclipse | A solar eclipse in which the Moon appears too small to cover the Sun completely, leaving a bright ring. |
| Solar eclipse alignment and phase | Sun-Moon-Earth, close to a straight line; possible at new moon. |
| Lunar eclipse alignment and phase | Sun-Earth-Moon, close to a straight line; possible at full moon. |
| Direct sunlight | Light striking more nearly perpendicular to a surface, concentrating its energy over a smaller area. |
| Indirect sunlight | Light striking at a shallower angle to a surface, spreading its energy over a larger area. |
| Summer Solstice | The time a hemisphere is tilted most toward the Sun; it marks that hemisphere's longest daylight period of the year. |
| Winter Solstice | The time a hemisphere is tilted most away from the Sun; it marks that hemisphere's shortest daylight period of the year. |
| Equinox | A time when neither hemisphere tilts toward the Sun and day and night are approximately equal at most locations. |
| Perihelion | The closest point to the Sun in an object's orbit; Earth reaches perihelion around early January. |
| Aphelion | The farthest point from the Sun in an object's orbit; Earth reaches aphelion around early July. |
| Tide | A periodic rise and fall in sea level caused mainly by the Moon's and Sun's gravitational effects, along with Earth's motion. |
| High tide | A local maximum in water level during a tidal cycle. |
| Low tide | A local minimum in water level during a tidal cycle. |
| Tidal bulge | An elongated region of higher water in the simplified tide model; there are bulges on the near and far sides along the Earth-Moon direction. |
| Tidal range | High-tide height minus low-tide height; it measures the difference between high and low water levels. |
| Spring Tides | Tides with a larger tidal range, generally near new and full moon when solar and lunar tidal effects reinforce one another. |
| Neap Tides | Tides with a smaller tidal range, generally near first and third quarter when solar and lunar tidal effects partly offset. |
| Spring tide geometry | The Sun, Earth, and Moon are approximately aligned; this can happen with the Moon on either side of Earth. |
| Neap tide geometry | The directions from Earth to the Sun and Moon form a right angle. |
Created by:
andrewpiraino
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