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Lunar Eclipse April 15th 2014  3:00AM Pictures below

Events in the orbit[edit]

By one astronomical convention, the four seasons are determined by flanges, the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere winter solstice occurs on about December 21, summer solstice is near June 21, spring equinox is around March 20 and autumnal equinox is about September 23. The axial tilt in the southern hemisphere is exactly the opposite of the direction in the northern hemisphere. Thus the seasonal effects in the south are reversed.

In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4 (for other eras, see precession and Milankovitch cycles). The changing Earth-Sun distance results in an increase of about 6.9%[2] in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. However, this effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.[3]

The Hill sphere (gravitational sphere of influence) of the Earth is about 1.5 Gm (or 1,500,000 kilometers) in radius.[4][nb 2] This is the maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects orbiting the Earth must be within this radius, otherwise they can become unbound by the gravitational perturbation of the Sun.

Orbital Characteristics
epoch J2000.0[nb 3]
aphelion 152,098,232 kilometres (94,509,460 mi)
1.01671388 AU[nb 4]
perihelion 147,098,290 kilometres (91,402,640 mi)
0.98329134 AU[nb 4]
semimajor axis 149,598,261 kilometres (92,956,050 mi)
1.00000261 AU[5]
eccentricity 0.01671123[5]
inclination 7.155° to Sun's equator
1.578690°[6] to invariable plane
longitude of the ascending node 348.73936°[1][nb 5]
argument of periapsis 114.20783°[1][nb 6]
period 365.256363004 days[7]
1.000017421 yr
average speed 29.78 kilometres per second (18.50 mi/s)[1]
107,200 kilometres per hour (66,600 mph)

The following diagram shows the relation between the line of solstice and the line of apsides of Earth's elliptical orbit. The orbital ellipse (with eccentricity exaggerated for effect) goes through each of the six Earth images, which are sequentially the perihelion (periapsis—nearest point to the Sun) on anywhere from 2 January to 5 January, the point of March equinox on 20 or 21 March, the point of June solstice on 20 or 21 June, the aphelion (apoapsis—farthest point from the Sun) on anywhere from 4 July to 7 July, the September equinox on 22 or 23 September, and the December solstice on 21 or 22 December. Note that the diagram shows an exaggerated representation of the shape of Earth's orbit. In reality, the actual path of Earth's orbit is not as eccentric as that portrayed in the diagram.

Seasons1.svg

Because of the axial tilt of the Earth in its orbit, the maximum intensity of sun rays hits the earth 23.4 degrees north of equator at the June Solstice (at the Tropic of Cancer), and the maximum intensity of sun rays hits the earth 23.4 degrees south of equator at the December Solstice (at the Tropic of Capricorn).

 

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