The four seasons are actually determined by the fact that the Earth is tilted 23.4° on its vertical axis, which is referred to as "axial tilt." This quirk in our orbit determines 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. The difference in distance between Earth's nearest point to the Sun in January and the farthest point from the Sun in July is 5 million kilometers. It is farthest from the Sun every year around July 4 (the "aphelion"). In fact, the Earth is farthest from the Sun in July and is closest to the Sun in January! Earth comes closest to the Sun every year around January 3 (the "perihelion"). Many people believe that the temperature changes because the Earth is closer to the Sun in summer and farther from the Sun in winter. Now you can talk to your kids about how Earth's elliptical orbit has nothing to do with our seasons. ![]() Thus, the speed of the planet increases as it nears the sun and decreases as it recedes from the sun.Īdd more interesting facts around the model in paint pens. This is Kepler's second law of planetary motion: a line between the sun and the planet sweeps equal areas in equal times. The months of June and July have a smaller portion of the arc of the orbit because the Earth travels slowest around that part of the ellipse farthest away from the Sun. ![]() December and January have a bigger portion of the arc of the orbit because the Earth travels fastest around that part of the ellipse that is closest to the Sun. Notice how the length of each calendar month varies around the orbit, i.e. Paint the months of the year around the orbit, making sure to put them in the correct spot based on where the Sun is located. This is Kepler's first law of planetary motion: all planets move about the Sun in elliptical orbits, having the Sun as one of the foci. Paint the styrofoam hemisphere to resemble the Sun and glue it to one of the two focus points used to make the elliptical orbit. Put your pencil in the loop of the string and stretch it to its limit and start drawing a large oval around the center of the board, keeping the string loop taunt at all times. Put your pushpins in these two spots and wrap a large loop of string around them. The foci always lie on the major (longest) axis, spaced equally each side of the center. The word foci (pronounced 'foe-sigh') is the plural of 'focus'. These are the ellipse's two focus points. ![]() In drawing the ellipse, you pick two points that are equidistant from the center of the board. This is a great time to involve your kids by making them draw the orbit. Our orbit ended up very "oval" looking and you can adjust this for your model, but I personally like that it drives home the point that Earth does not have a circular orbit around the Sun. Earth's orbit has an eccentricity of less than 0.02, which means that it is very close to being circular. If it is close to one, the ellipse is long and slender. If a planet's eccentricity is close to zero, then the ellipse is nearly a circle. In describing the nature of elliptical orbits, scientists use a factor known as "eccentricity", which is expressed in the form of a number between zero and one. This means there is one point in the orbit where Earth is closest to the Sun, and another where Earth is farthest from the Sun. Earth's orbit is not a perfect circle either- it is elliptical, or slightly oval-shaped. The three separate orbits are spaced equally around the Earth, but share a common ground track.The gravitational pull of the Sun is responsible for the Earth's orbit around the Sun. The longitude above which the satellites dwell at apogee in the small loop remains relatively constant as Earth rotates. Sirius Satellite Radio used inclined HEO orbits, specifically the Tundra orbits, to keep two satellites positioned above North America while another satellite quickly sweeps through the southern part of its 24-hour orbit. Geostationary orbits cannot serve high latitudes due to their altitude from ground sites being too low. This makes these elliptical orbits useful for communications satellites. Bodies moving through the long apogee dwell appear to move slowly, and remain at high altitude over high-latitude ground sites for long periods of time. Such extremely elongated orbits have the advantage of long dwell times at a point in the sky during the approach to, and descent from, apogee. Examples of inclined HEO orbits include Molniya orbits, named after the Molniya Soviet communication satellites which used them, and Tundra orbits. ![]() A highly elliptical orbit (HEO) is an elliptic orbit with high eccentricity, usually referring to one around Earth.
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