What’s Up in the Sky – April, 2015
A Special Lunar Eclipse
What is a tetrad? I did not know either until I started researching this month’s article. You may already be aware of the fact that a total lunar eclipse will occur in the pre-dawn hours of April 4. But you probably are not aware that this will be the third eclipse of the current tetrad. Which brings us back to the question, “what”?
A tetrad is a series of four consecutive total lunar eclipses. Now, you probably are wondering why that is unusual. After all, lunar eclipses generally occur about twice a year, on average. But remember, not all lunar eclipses are total. There are actually three types of lunar eclipses: total, partial, and penumbral.
A penumbral eclipse occurs when the Moon passes through the faint outskirts of Earth’s shadow. An observer on the Moon during such an event would see only part of the Sun covered by the Moon. Thus, penumbral eclipses are often so subtle that they are not even noticed.
A partial eclipse is more pronounced because part, but not all, of the Moon passes through the core of Earth’s shadow so only part of the Moon becomes darkened.
A total eclipse occurs when the entire Moon passes through the darkest part of Earth’s shadow and the entire face of the Moon turns sunset-red for an extended period of time.
Most of the time lunar eclipses occur in random order. We could have a total followed by a penumbral, followed by another total or even partial – the sequence is always unique. But when four eclipses in a row are all total, the sequence is called a tetrad.
This is the second of eight tetrads in the 21st century making it a rather common event. But that has not always been the case. During the three hundred years from 1600 to 1900 there were none.
A unique feature of the current tetrad is that all four eclipses have been and will be visible from North America. The first two occurred in April and October of last year and the fourth one will be on September 27/28 of this year. And even though Saturday’s will begin before dawn and the Moon will set during totality, the one next September will be much easier to witness with totality occurring around midnight.
So now you have a new Scrabble word to use when you are enjoying what’s up in the sky.
This month in history:
April 1: Comet Hale-Bopp nearest Sun – 1997
April 9: NASA selects original seven Mercury astronauts – 1959
April 12: Yuri Gagarin becomes first human in space – 1961
April 12: Columbia is first space shuttle to be launched – 1981
April 17: Apollo 13 returns to Earth – 1970
April 20: Apollo 16 lands on the Moon – 1972
April 24: China becomes the fifth nation to launch its own satellite – 1970
April 28: Eugene Shoemaker is born – 1928