Mon Jul 5, 2021 EARTH AT APHELION
Today, Monday, July 5th, at 6:27 pm, the earth will reach aphelion – that’s the point in our planet’s slightly elliptical orbit where it’s farthest from the sun. On average, we're about 93 million miles from the sun, but right now we are roughly 94 and a half million miles out. So how come we're having summer? Well, not everyone on earth is experiencing summer; winter has just begun for folks south of the equator. Our seasons aren't caused by any variation in the earth-sun distance; after all, that extra million and a half miles only makes for a tiny 2% difference. Temperature changes occur because our planet is tilted over a little, about 23 and a half degrees, from straight up and down. During summer in the northern hemisphere, the top half of earth leans inward, which puts the sun higher in our sky, and causes summer; in the winter the top half of earth leans away from the sun, putting it lower in our sky, which cools things down.
Tue Jul 6, 2021 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.
Wed Jul 7, 2021 TANABATA DAY: VEGA AND ALTAIR
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 it 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.”
Thu Jul 8, 2021 DECLARATION OF INDEPENDENCE
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.
Fri Jul 9, 2021 PRINCIPIA
On July 6 in the year 1686, Principia Mathematica was published in England. Principia was Isaac Newton’s great book on gravity and motion, which became a major breakthrough for our understanding of how the Universe works. His three laws of motion – inertia; force equals mass times acceleration; and action-reaction, plus the relationship between gravity, mass and distance, are still in use today, showing us how we can send rockets to the moon and beyond. Kind of a shame he didn’t pay for the printing of his own book. Edmond Halley paid for its publishing, because he wanted it to help him work out comet orbits. Halley tried to get the Royal Society to pay for it, but they’d tied up all their money in a beautiful book, the History of Fishes, which they weren’t able to sell.
