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Skywatch for the week of July 6, 2020


On the 4th of July in the year AD 1054, a bright star suddenly appeared in the eastern predawn sky. It was off in the direction of the constellation Taurus, just behind the forward horn tip of the bull. For the next several weeks this new star, this “nova,” was so bright that it could even be seen after sunrise, in the daytime! And then as summer drew to a close, the star faded out of sight and was seen no more. In the western world there is apparently no written record of this star’s appearance: either no one was looking up then, or more likely, the skies were overcast throughout the star’s appearance. But in the east, Chinese astronomers made note of this “guest star,” as they called it, and that’s how we know about it today. If you’re out before sunrise this month, aim your telescope at that part of space behind the forward horn tip of Taurus, and you’ll find the Crab nebula, the exploded remains of a supernova - cosmic fireworks from nearly a thousand years ago.



Today is Tanabata Day in Japan, marking the reunion of the weaver princess and the cowherd. This far-eastern story is over a thousand years old.  The Jade Emperor’s daughter, Tanabata or Chih-Nu, loved a herdsman, Niu Lang. The father disapproved, and so he placed them up into the sky; Chih-Nu became the star Vega, and Niu Lang is the star Altair - both stars are well-placed in the eastern sky after sunset tonight. The Emperor then set Tien-Ho, the great Celestial River to separate them. Tien-Ho is the Milky Way, which when the skies are dark, you can see runs between these two stars. But on the seventh day of the seventh month, if skies are clear, magpies gather and with their wings form a living bridge across the Milky Way, so Chi-Nu and Niu Lang can be together once more. Part of a traditional poem recited at this time goes, “the stars twinkle on the gold and silver grains of sand... The stars twinkle, and there they will watch us.”



In the year 1769 an observatory was built in Philadelphia, just a couple of hundred feet south of Independence Hall. It had been built so that astronomers could observe a transit of the planet Venus that year. A transit occurs when either Mercury or Venus passes directly between the earth and the sun; with protective filters, we see those planets as small, dark round dots against the sun’s face. Transits of Venus are rare; they occur in pairs every hundred and twenty years. Seven years after colonial astronomers saw this transit the observatory was still there, and its balcony made an excellent platform for the first public reading of the Declaration of Independence, on this day, July 8th, in the year 1776. During the Revolutionary War, the Philadelphia observatory housed British troops who occupied the city. And not too many years after the end of the war, the observatory fell into disuse, and sadly, is no longer there.


Thu Jul 9, 2020     HENRIETTA LEAVITT

On July 6, 1868, the American astronomer Henrietta Leavitt was born. She worked at the Harvard Observatory, and while cataloging a class of stars known as Cepheid variables, the first example of which came from the fourth-brightest star in the constellation Cepheus the King, Leavitt analyzed the light curves of various Cepheids. Variable stars change their brightnesses over time; this is caused by the star’s expanding and contracting as it reaches the end stages of its life. When the star expands, it becomes brighter, when it contracts, it dims a bit. Henrietta Leavitt discovered that there was a relationship: Cepheid variable stars that were intrinsically brighter, or larger, than others, took longer to go from bright to dim to bright again. This made it possible to figure out how far away distant galaxies were, and gave us a much larger measuring stick to determine how far away things are in the Universe.


Fri Jul 10, 2020    FAREWELL, SKYLAB

Forty-one years ago, Skylab burned up as it re-entered earth’s atmosphere. It was America’s first space station, built from hardware left over from the Apollo moon missions. The station was made from the Saturn 5 rocket’s third stage, and was launched in 1973. Over the next year, it was visited by three different Skylab crews, providing these astronauts with a base for observing the earth and the sun and the stars. And it provided lessons that would help people who flew on future missions, such as those aboard the current space station, during long-duration flights. There was even a race track reminiscent of the one seen in the movie 2001: A Space Odyssey, where astronauts could run laps around Skylab’s inner circumference! One of my duties when I interned at the Hayden Planetarium was to provide daily updates on the anticipated re-entry time of Skylab. It was indeed a sad day, July 11, 1979, when it broke up over the South Pacific Ocean and Australia.