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ELLIPTICAL ORBIT

The reasons for this yearly variation in the apparent motion of the Sun are twofold. The

first reason has to do with the fact that the Earth's orbit is not a perfect circle, but is elliptical with

the Sun being nearer one end of the ellipseThe speed of the Earth in this elliptical orbit varies from a minimum at the farthest

distance to a maximum at the closest distance of the Earth to the Sun. The second reason for theyearly variation has to do with the fact that the Earth's equator is inclined to the plane of the

Earth's orbit around the Sun. These two effects are explained in the following paragraphs.

The Earths orbit around the Sun has many interesting characteristics. irst, the speed of

our orbit is !"#,""" $m%h. The planet travels &" million $m during one orbit. The Earth

completes one orbit every ()*.++!&& mean solar days that might help explain the need for a

leap year-. The planets distance from the Sun varies as it orbits. ctually, the Earth is never the

same distance from the Sun from day to day. /hen the Earth is closest to the Sun it is said to be

at perihelion. This occurs around 0anuary (rd at a distance of !1,","1 $m. /hen it is at its

furthest distance from the Sun, Earth is said to be at aphelion. That happens around 0uly th at a

distance of !*+,"&1,1"! $m.

Eccentricity of the earths orbitEarth's orbital eccentricity is less than "."+. The orbit of2lutois the most eccentric of

any planet in our Solar System.2luto'sorbital eccentricity is almost ".+*. 3any cometshave

extremely eccentric orbits. 4alley's 5omet,for instance, has an orbital eccentricity of almost

".&16The Sun is not at the center of an elliptical orbit. 7t is a little off to one side, at a point

called a 8focus8 of the ellipse. 9ecause of this offset the planet moves closer to and further away

from the Sun every orbit. The close point in each orbit is calledperihelion. The far away point is

called aphelion. 7f an orbit has a large eccentricity, the difference between the perihelion distance

and the aphelion distance will also be large. Earthis only (: further from the Sun at aphelion

than it is at perihelion. 2luto's aphelion distance from the Sun is )): greater than its perihelion

distance.

Concept of the earths axial rotation giving day and night

; s the earth spins on its axis, only one half of the earths surface faces the sun. Therefore

only that half receives the light and heat of the sun, this half is experiencing daylight and

the half that is on the other side < the dar$ side = it is >ighttime.

; s the earth rotates more areas come within daylight and other area on the days edge

move into dar$ness or night.

S?@ST75E

A solsticeis an astronomical event that occurs twice each year asthe Sunreaches its highest or lowest excursion relative to thecelestial equatoronthe celestial sphere. The solstices, together with theequinoxes, are connected with the

seasons. In many cultures the solstices mark either the beginning or the midpoint ofwinter and summer.

!"I#$%Two times of the year when night and day are about the same length. The Sun is crossing the

Equator an imaginary line around the middle of the Earth- and it is an equal distance from the

>orth 2ole and the South 2ole.

http://www.windows2universe.org/pluto/statistics.htmlhttp://www.windows2universe.org/pluto/statistics.htmlhttp://www.windows2universe.org/our_solar_system/solar_system.htmlhttp://www.windows2universe.org/our_solar_system/solar_system.htmlhttp://www.windows2universe.org/pluto/pluto.htmlhttp://www.windows2universe.org/pluto/pluto.htmlhttp://www.windows2universe.org/comets/comets.htmlhttp://www.windows2universe.org/comets/comets.htmlhttp://www.windows2universe.org/comets/Halleys_comet.htmlhttp://www.windows2universe.org/comets/Halleys_comet.htmlhttp://www.windows2universe.org/physical_science/physics/mechanics/orbit/perihelion_aphelion.htmlhttp://www.windows2universe.org/physical_science/physics/mechanics/orbit/perihelion_aphelion.htmlhttp://www.windows2universe.org/earth/earth.htmlhttp://en.wikipedia.org/wiki/Sunhttp://en.wikipedia.org/wiki/Celestial_equatorhttp://en.wikipedia.org/wiki/Celestial_spherehttp://en.wikipedia.org/wiki/Equinoxhttp://en.wikipedia.org/wiki/Equinoxhttp://www.windows2universe.org/pluto/statistics.htmlhttp://www.windows2universe.org/our_solar_system/solar_system.htmlhttp://www.windows2universe.org/pluto/pluto.htmlhttp://www.windows2universe.org/comets/comets.htmlhttp://www.windows2universe.org/comets/Halleys_comet.htmlhttp://www.windows2universe.org/physical_science/physics/mechanics/orbit/perihelion_aphelion.htmlhttp://www.windows2universe.org/physical_science/physics/mechanics/orbit/perihelion_aphelion.htmlhttp://www.windows2universe.org/earth/earth.htmlhttp://en.wikipedia.org/wiki/Sunhttp://en.wikipedia.org/wiki/Celestial_equatorhttp://en.wikipedia.org/wiki/Celestial_spherehttp://en.wikipedia.org/wiki/Equinox

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