The Selenelion Or Horizontal Lunar Eclipse

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If you are interested in experiencing something that very few other eclipse watchers have seen, this event in 2003 may provide you with an opportunity. If you were in one of those zones where the eclipse is in progress at moonrise or moonset you have the peculiar chance to be able to see both the Sun and the eclipsed Moon in the sky at the same time, with a quick twist of the head. An eclipse occurs when the two celestial orbs are 180 degrees apart, with the Earth in between. The refraction (or bending) of light beams in the Earth's atmosphere, however, makes it possible to see both at once. Geometrically they may both be below the horizon, but the refraction by about half a degree makes this double appearance possible. In order to witness this you need to go to as high an altitude as possible and have a clear distant horizon to both the east and west. You have only a fleeting chance lasting a few minutes.

In general this is possible only with the partially eclipsed

Moon, because when the eclipse is total the Moon is simply too dark to see when it is also right on the horizon. At that time you are looking through such a thickness of atmosphere that the weak light from the totally eclipsed orb is attenuated to leave almost nothing. The best chance is when there is still a thin crescent of the lunar disk illuminated by the Sun, meaning between contact points U1 and U2, or between U3 and U4 in Figure 2-5 indicates that the Hawaiian Islands are a candidate location, although many parts of the western United States will also provide an opportunity.

This phenomenon is called a "horizontal eclipse" or, from a French term, a "selenelion." There is evidence that the Babylonians noted an occurrence of this peculiarity in 1713 B.C. In modern times the first record seems to date from 1590, when the great astronomer Tycho Brahe saw a selenelion from his observatory on the island of Ven that lies in the strait between Sweden and Denmark. Five more such events were recorded from Europe over the next century (see Figure 2-6 for an example from 1666), but none in the 1700s and only one during the 1800s. The next record was not until 1975, when Allan Fries noted a selenelion from an island in the sound off Everett, Washington. The first photograph of a selenelion was not obtained until July 16, 1981, when professional

FIGURE 2-5. Details of the total lunar eclipse that will occur on May16, 2003. Various types of pertinent data are shown, such as the timings for different contact points and the locations from where the eclipse may be viewed. Note the angle between the path of the Moon and the ecliptic: the eclipse occurs near the descending node. Also note that the nodal passage occurs well after the eclipse has finished, an example of the idea of the ecliptic limits.

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Selenelion

FIGURE 2-6. A sketch of the circumstances of the lunar eclipse observed on June 16, 1666. Prince Leopold of Florence instructed his astronomers to go to the island of Gorgona, 30 miles off the Italian coast near Livorno, in order to record what was seen. The flat Mediterranean Sea provided their horizon to the west where the Sun was setting. By gaining some altitude the distant Appenines were visible low in the east where the Moon was rising, and they were able to witness both the Sun and the eclipsed Moon in the sky at the same time. Such an observation is known as a "selenelion."

FIGURE 2-6. A sketch of the circumstances of the lunar eclipse observed on June 16, 1666. Prince Leopold of Florence instructed his astronomers to go to the island of Gorgona, 30 miles off the Italian coast near Livorno, in order to record what was seen. The flat Mediterranean Sea provided their horizon to the west where the Sun was setting. By gaining some altitude the distant Appenines were visible low in the east where the Moon was rising, and they were able to witness both the Sun and the eclipsed Moon in the sky at the same time. Such an observation is known as a "selenelion."

astronomer William Sinton permanently recorded one from the Mauna Kea Observatory in Hawaii.

That brings us to the present. A few days before the lunar eclipse on January 9, 2001, I realized that a selenelion might be seen from Adelaide in South Australia, where I had lived for some years. So I alerted friends in the local astronomical society, suggesting that they might try to catch a glimpse from Mount Lofty, the tallest hill on the eastern fringe of the city. I knew that to the west they would look out over the sea, while to the east their view would be over the extensive plains through which the River Murray flows. And in January, at the start of the southern summer, the sky was almost certain to be clear. A small group rose early and climbed not only that hill, but also the fire-spotting tower at the summit, from where they were afforded an excellent view. The result is shown in Figure 2-7.

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Responses

  • Jan
    When is the next selenelion eclipse?
    12 months ago

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