4 fall september equinox occurs around september 22

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4. fall (September) equinox: occurs around September 22, the moment when the Northern
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Hemisphere first starts to be tipped away from the Sun. Equinoxes occur on the only two days when sunlight falls equally on both hemispheres. Each equinox and solstice marks the first day of a season. For people in the Southern Hemisphere, winter begins when Earth is at the orbital point usually called the summer solstice. (opposite of equinox and solstice titles) At higher latitudes, seasonal variations around the times of the solstices are more extreme. Ex. Vermont has much longer summer days and winter nights than Florida. Precession is a gradual wobble that alters the orientation of Earth's axis in space. While today the axis points toward Polaris, 13,000 years from now precession will render Vega to be the star closest to truth north. Precession is caused by gravity's effect on a tilted rotating object that is not a perfect sphere. Precession does not change the amount of the axis tilt (which stays close to 23 ½ degrees) * the latitude at which the sun is directly overhead is called the Tropic of Cancer The spinning (rotating) Earth precesses because of gravitational tugs from the Sun and Moon. Earth tends to keep rotating around the same axis, therefore gravity does not succeed in changing earth's axis tilt and instead only makes the axis precess. The moon orbits the earth about 27 1/3 rd days. Lunar phases of the moon change based on position relative to the Sun as it orbits Earth. 1. Phases from new to full are said to be waxing, or “increasing” 2. Phrases from full to new are waning, or “decreasing” 3. Phrases just before and after new moon are “crescent” 4. Phrases just before and after full moon are “gibbous” The moon rotates on its axis in the same amount of time that it takes to orbit Earth, a trait called synchronous rotation. The dark portion of the lunar face is not totally dark, ashen light or earth shine is when the reflected light from the earth illuminates the outline of a crescent moon. An eclipse is when the sun, earth, and moon fall into a straight line. A lunar eclipse occurs when Earth lies directly between the sun and moon, so that the earth's shadow falls on the moon. A solar eclipse occurs when the Moon lies directly between the Sun and Earth, so that the moon's shadow falls on the earth. The two points in each orbit at which the Moon crosses the surface are called the nodes of the Moon's orbit. Two conditions for an eclipse to occur: 1. The phase of the moon must be full (for a lunar eclipse) or new (for a solar eclipse). 2. The new or full moon must occur during one of the period when the nodes of the Moon's orbit are aligned with the Sun and Earth.
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