A line that connects a planet to the sun sweeps out equal areas in equal times.-physicsknow

The Law of Areas-PhysicsKnow

The orbital radius and angular velocity of the planet in the elliptical orbit will vary. This is shown in the animation: the planet travels faster when closer to the sun, then slower when farther from the sun. Kepler's second law states that the blue sector has constant area.

Kepler-second-law

This is one of Kepler's laws.This empirical law discovered by Kepler arises from conservation of angular momentum. When the planet is closer to the sun, it moves faster, sweeping through a longer path in a given time.

Developing Kepler's Law of Areas-PhysicsKnow Law of Areas: Considering the area of an elliptical orbit, an infinitesemal area element can be expressed as Integrating over r from the focus outward gives So the time rate of change of the area swept out is The velocity is in the plane of the ellipse and can be divided into radial and angular components: Note that the angular component is proportional to the rate of change of area so that Note that the radius r and the angular velocity are perpendicular to each other so that their product is equal to the magnitude of their vector product. But it was shown in the development of the Law of Orbits that this vector product is proportional to the angular momentum L. It was also shown there that the angular momentum L is a constant, so that establishes that the rate of change of area is a constant for all parts of the orbit, the Law of Areas.

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Solar eclipse of August 21, 2017-PhysicsKnow

Solar eclipse of August 21, 2017-PhysicsKnow

Solar eclipse of August 21, 2017-PhysicsKnow

On Monday, August 21, 2017, a total solar eclipse will be visible within a band across the entire contiguous United States. This eclipse will only be visible in other countries as a partial eclipse.

A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby totally or partly obscuring the image of the sun for a viewer on Earth. A total solar eclipse occurs when the moon's apparent diameter is larger than the sun's, blocking all direct sunlight, turning day into darkness. Totality occurs in a narrow path across Earth's surface, with the partial solar eclipse visible over a surrounding region thousands of kilometers wide.

The last time a total solar eclipse was visible across the entire contiguous United States was during the June 8, 1918 eclipse, and not since the February 1979 eclipse has a total eclipse been visible from anywhere in the mainland United States. The path of totality will touch 14 states (although a partial eclipse will be visible in all fifty states),  and 16% of the area of the United States. The event will begin on the Oregon coast as a partial eclipse at 9:06 a.m. PDT on August 21, and will end later that day as a partial eclipse along the South Carolina coast at about 4:06 p.m. EDT.

There are expected to be logistical problems with the influx of visitors, especially for smaller communities.There have also been problems with counterfeit eclipse glasses being sold.

Future total solar eclipses will cross the United States in April 2024 (12 states) and August 2045 (10 states), and annular solar eclipses—meaning the apparent size of the Moon is smaller than that of the Sun—will occur in October 2023

 (9 states) and June 2048 (9 states).

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