Unit 9: Earth Cycles Vocabulary Axis Imaginary line about which an object rotates. Rotate To turn or cause to turn about an axis or a center. Revolve To move in an orbit. Counterclockwise In a direction opposite to that in which the hands of a clock rotate. Rotational Axis The Earth is rotating around an imaginary axis at a 23.5° tilt. It points in the same direction relative to the stars, so that the North Pole points towards the star Polaris (North Star). Vernal Equinox Earth reaches a point where the tilt is not toward or away from the Sun, and the lengths of day and night are the same all over Earth. March 21st. Summer Solstice Earth’s northern hemisphere is tilted toward the Sun and days become longer and warmer. June 21st. Autumnal Equinox Earth reaches a point where the tilt is not toward or away from the Sun, and the lengths of day and night are the same all over Earth. September 22nd. Winter Solstice Earth reaches a point when the northern hemisphere is tilted away from the sun and the hours of daylight are the shortest. December 21st. Eclipse The partial or complete hiding from view of an astronomical object, such as the Sun or Moon, when another astronomical object comes between it and the observer. Solar Eclipse When the Moon blocks the Sun or a part of it. Lunar Eclipse When the Earth casts a shadow on the Moon. New Moon When the Moon is between the Earth and the Sun; the illuminated portion is on the backside we can’t see. Waxing Crescent During this phase, part of the Moon is beginning to show. We say that the Moon is "waxing" because each night a little bit more is visible for a little bit longer. First Quarter Moon Comes a week after new moon; rises at noon and is high overhead at sunset, then sets around midnight. Waxing Gibbous When most of the Moon is visible we say it is a Gibbous Moon. During this phase, the Moon remains in the sky most of the night. Full Moon When we can observe the entire face of the Moon, we call it a Full Moon. The Moon is opposite the Earth and the Sun. Waning Gibbous Instead of seeing more of the Moon each night, we begin to see less and less of the Moon each night. This is what the word "waning" means. Third Quarter Moon Comes about three weeks after new moon; rises around midnight, appears at its highest in the sky around dawn, and sets at noon. Waning Crescent Each night less of the Moon is visible for less time. Gravitational Attraction Force of attraction between all masses in the universe, especially the attraction of the Earth’s mass for bodies near its surface. Neap Tides Tides that are the least extreme; happen twice a month, at first and last quarter moon phases. Spring Tides Tides that are most extreme; occur twice a month, at full and new moon phases. High Tide The tide when the water is at its greatest height. Low Tide The tide at its lowest level at a particular time and place.