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  • Earth at Perihelion

    Reggie Jones
    For planet Earth, perihelion came & went, on Tuesday January 2nd, 2024, at 4:48 PM PST, 7:38 PM EST. That’s 00:38 (12:38AM) UTC on the following day, Wednesday January 3rd, by universal coordinated time (UTC). Perihelion is the auspicious moment in the annual orbital adventure of Planet Earth, when the distance between Earth and sun is the smallest it will be all year.
    The minimum center-to-center Earth-sun distance was (according to the JPL Horizons ephemeris calculator) 0.98330694871998 astronomical units (AU), which translates into 91,404,091.215 miles (at 92,955,807.273 miles/AU). At perihelion it will take light 8.17791452 minutes (8 minutes 10.675 seconds) to make the Journey from the sun to Earth.
    After perihelion, Earth will begin to move away from the sun, until it reaches aphelion, its farthest distance from the sun, on Thursday July 4th 2024, at 9:06 PM PDT, or 12:06 AM EDT on Friday July 5th EDT. And that’s 05:06 (5:06 AM) UTC. At that time the center-to-center Earth-sun distance will be 1.01672552876718 AU, which translates into 94,510,542.301 miles. At aphelion, it will take light 16.676 seconds longer to reach Earth, than it did at perihelion.
    So, as Northern Hemisphere summer is in full swing, Earth will be 3,106,451.086 miles farther from the sun, than it was in mid-winter. So if you didn’t already know, now you do know, that the seasons are not related to the Earth-sun distance at all. They are instead related to the angle of the spin axis of Earth, compared to the line between Earth and the sun, the obliquity, in astronomy jargon.
    The first diagram shows that Earth’s orbit is not circular, but is instead elliptical, its ellipticity exaggerated for educational clarity. In fact, Earth does not go around the sun. Rather, Earth, sun, and everything else in the solar system (planets, comets asteroids & etc.) orbit around the barycenter (gravitational balance point) of the solar system. But since everything is moving around, so does the barycenter, and as a result, nothing in the solar system follows a truly fixed orbit.
    So, the second diagram shows how the solar system barycenter moves around, compared to the sun itself (the yellow dotted circle at the center of the image). As you can see, sometimes the barycenter is as far a one solar radius outside the surface of the sun. So a distant observer would see the sun wobble, and thereby detect the presence of Jupiter, which is more massive that everything else in the solar system combined, absent the sun.
    https://aa.usno.navy.mil/data/Earth_Seasons (Earth's Seasons - Equinoxes, Solstices, Perihelion, and Aphelion - U.S. Naval Observatory, Data Services)
    https://aa.usno.navy.mil/faq/seasons_orbit (The seasons and Earth’s orbit - U.S. Naval Observatory, information center)
    https://aa.usno.navy.mil/faq/UT (Universal time - U.S. Naval Observatory, information center)
    https://en.wikipedia.org/wiki/Perihelion_and_aphelion (perihelion & aphelion - Wikipedia)
    https://www.farmersalmanac.com/aphelion-and-perihelion (perihelion & aphelion - Farmer’s Almanac)
    https://www.almanac.com/content/what-aphelion-and-perihelion (perihelion & aphelion - Old Farmer’s Almanac)
    https://www.timeanddate.com/.../perihelion-aphelion... (perihelion & aphelion - Time and Date)
    https://earthsky.org/.../earth-comes-closest-to-sun.../ (perihelion & aphelion - EarthSky)
    https://www.weather.gov/fsd/season (What causes the seasons? - National Weather Service, Sioux Falls, South Dakota)
    https://www.weather.gov/lmk/seasons (Why do we have seasons? - National Weather Service, Louisville, Kentucky)
    https://spaceplace.nasa.gov/seasons/en/ (What causes the seasons? - NASA Space Place)
    https://www.calacademy.org/.../why-do-we-have-different... (Why do we have different seasons? - California Academy of Sciences, Habitat Earth in the classroom)
    https://en.wikipedia.org/wiki/June_solstice (June Solstice - Wikipedia)
    https://en.wikipedia.org/wiki/December_solstice (December solstice - Wikipedia)
    Originally posted by Tim Thompson, Retired Senior Astronomer NASA JPL



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