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# Perihelion and aphelion

The perihelion of any orbit of a celestial body about the Sun is the point where the body comes nearest to the Sun. [1]

## Apsis

An apsis (ἁψίς; plural apsides, Greek: ἁψῖδες) is an extreme point in the orbit of an object.

## Astronomical object

An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that exists in the observable universe.

## Axial tilt

In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane.

## Circle

A circle is a simple closed shape.

## Earth

Earth is the third planet from the Sun and the only astronomical object known to harbor life.

## Ellipse

In mathematics, an ellipse is a curve in a plane surrounding two focal points such that the sum of the distances to the two focal points is constant for every point on the curve.

## Johannes Kepler

Johannes Kepler (December 27, 1571 – November 15, 1630) was a German mathematician, astronomer, and astrologer.

## Milankovitch cycles

Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years.

## NASA

The National Aeronautics and Space Administration (NASA) is an independent agency of the executive branch of the United States federal government responsible for the civilian space program, as well as aeronautics and aerospace research.

## Orbit

In physics, an orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet.

## Orbital eccentricity

The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle.

## Perturbation (astronomy)

In astronomy, perturbation is the complex motion of a massive body subject to forces other than the gravitational attraction of a single other massive body.

## Season

A season is a division of the year marked by changes in weather, ecology, and amount of daylight.

## Solar System

The Solar SystemCapitalization of the name varies.

## Solstice

A solstice is an event occurring when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere.

## Summer

Summer is the hottest of the four temperate seasons, falling after spring and before autumn.

## Sun

The Sun is the star at the center of the Solar System.

## United States Naval Observatory

The United States Naval Observatory (USNO) is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation and Timing (PNT) for the United States Navy and the United States Department of Defense.

## Washington State University

Washington State University (WSU) is a public research university in Pullman, Washington, in the Palouse region of the northwest United States. Founded in 1890, WSU (colloquially "Wazzu") is a land-grant university with programs in a broad range of academic disciplines. It is ranked in the top 140 universities in America with high research activity, as determined by U.S. News & World Report. With an undergraduate enrollment of 24,470 and a total enrollment of 29,686, it is the second largest institution of higher education in Washington state behind the University of Washington. The university also operates campuses across Washington known as WSU Spokane, WSU Tri-Cities, WSU Everett and WSU Vancouver, all founded in 1989. In 2012, WSU launched an Internet-based Global Campus, which includes its online degree program, WSU Online. These campuses award primarily bachelor's and master's degrees. Freshmen and sophomores were first admitted to the Vancouver campus in 2006 and to the Tri-Cities campus in 2007. Enrollment for the four campuses and WSU Online exceeds 29,686 students. This includes 1,751 international students. WSU's athletic teams are called the Cougars and the school colors are crimson and gray. Six men's and nine women's varsity teams compete in NCAA Division I in the Pac-12 Conference. Both men's and women's indoor track teams compete in the Mountain Pacific Sports Federation.

## Winter

Winter is the coldest season of the year in polar and temperate zones (winter does not occur in the tropical zone).

## References

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