![]() ![]() Less than 2% larger than an average perigean full moon.Let's go back to November 13-14, 2016, when the extreme perigean full moon’s radius and diameter were: Something that’s out of the ordinary: Having a full moon at the same time as an extreme perigee.Īn extreme perigean full moon’s angular radius and diameter will appear slightly larger than it does at other full moons. Extreme perigees and apogees, or the most distant point in the orbit, happen on a predictable basis. The shape of the Moon’s orbit changes over time (thanks to the gravitational influence of the Sun and the other planets). ![]() Some lunar perigees are closer than others. Except for the small change in distance between their centers due to the Moon’s orbital motion in those 6.5 hours, the observer’s decrease in distance from the Moon is demonstrated by the pair of photographs. At that time it was almost 4,000 miles closer to the observer because Earth’s rotation has carried the observer more directly under the Moon Earth’s center was now farther from the Moon than the observer was. At the time of the second picture, Earth had rotated about a quarter-turn and the Moon was highest in the sky. ![]() At that time, the Moon’s center was about the same distance from Earth’s center and from the observer. The lines across the pair of images show the apparent difference in size: The rising moon is smaller because it was further away when it was rising. The right image was taken with the Moon near its maximum altitude that night, at about 12:30 a.m. ![]() local time when it had cleared the local, mountainous horizon. The figure’s left image was taken after moonrise, at about 6 p.m. The full-frame of each picture is reproduced. Note the lower blue line cutting across the Moon’s south pole on the right image. On the left, the Moon was nearer the horizon, and on the right, the Moon was very high in the sky. Which of these views of the Moon is a supermoon? Both! This is a size comparison of the Moon on the supermoon night of November 13-14, 2016. Take a look at the lunar images from November 2016 below: The term gives preference to the geometric alignment of Sun-Earth-Moon and allows the occurrence of perigee into a wider time period than the actual instant of perigee (up to about two weeks, which is almost half of the Moon’s orbit).įor those looking carefully, there are several things worth noting. The term “supermoon” was coined in 1979 and is often used to describe what astronomers would call a perigean ( pear-ih-jee-un) full moon: a full moon occurring near or at the time when the Moon is at the closest point in its orbit around Earth. When a full moon appears at perigee it is slightly brighter and larger than a regular full moon – and that's where we get a "supermoon.Its closest point is the perigee, which is an average distance of about 226,000 miles (363,300 kilometers) from Earth.The farthest point in this ellipse is called the apogee and is about 253,000 miles (405,500 kilometers) from Earth on average.The Moon orbits Earth in an ellipse, an oval that brings it closer to and farther from Earth as it goes around.So what's so special about a supermoon? For the interested observer, there's plenty to see and learn. " This piece was inspired by Sutro Baths, a seaside bathhouse located in San Francisco established in the 1920s where waters change with the tides.įor the Coast and Geodetic Survey, Marmer wrote " Tides and Currents in New York Harbor ", " Coastal Currents along the Pacific Coast of the United States ", " Tidal Datum Planes ", and " Chart Datums ".A supermoon occurs when the Moon’s orbit is closest (perigee) to Earth at the same time the Moon is full. Scientific American has described Segal's Tidal Datum as " spectacular " with " unbeatable beauty and wonder. Usually, the starting reference point is a tide gauge, so at that point the geodetic and tidal datums might match, but due to sea level variations, the two scales may not match elsewhere. Sea level does not remain constant throughout geological time, and so tidal datums are less useful when studying very long-term processes.ĭuring his long career, Marmer gained international recognition as an expert on tides, currents, tidal datum planes, and related oceanographic subjects. This is a tidal datum which is described as the arithmetic mean of the hourly water elevation taken over a specific 19 years cycle. All values were recorded at the Punta della Salute ( Venice ) station and refer to the 1897 tidal datum point. ![]()
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