                        Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our
      fascinating universe is featured, along with a brief explanation
                    written by a professional astronomer.

                               2021 January 11

                             Moon Phases in 2021
    Video Credit: Data: Lunar Reconnaissance Orbiter ; Animation: NASA's
                      Scientific Visualization Studio;
    Music: Brandenburg Concerto No4-1 BWV1049 (Johann Sebastian Bach), by
                        Kevin MacLeod via Incompetech

   Explanation: What will the Moon phase be on your birthday this year? It
   is hard to predict because the Moon's appearance changes nightly. As
   the Moon orbits the Earth, the half illuminated by the Sun first
   becomes increasingly visible, then decreasingly visible. The featured
   video animates images taken by NASA's Moon-orbiting Lunar
   Reconnaissance Orbiter to show all 12 lunations that appear this year,
   2021. A single lunation describes one full cycle of our Moon, including
   all of its phases. A full lunation takes about 29.5 days, just under a
   month (moon-th). As each lunation progresses, sunlight reflects from
   the Moon at different angles, and so illuminates different features
   differently. During all of this, of course, the Moon always keeps the
   same face toward the Earth. What is less apparent night-to-night is
   that the Moon's apparent size changes slightly, and that a slight
   wobble called a libration occurs as the Moon progresses along its
   elliptical orbit.

    APOD online webinar January 12: Free registration, hosted by Amateur
                    Astronomers Association of New York.
                      Tomorrow's picture: folklore sky
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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                      A service of: ASD at NASA / GSFC
                             & Michigan Tech. U.

