For those watching the sky in North America tonight, you might notice that something is missing: the moon! This is because July 31st marks a lunar event called the “black moon” which is the second new moon that occurs in a calendar month. A new moon is the phase of the moon where it is invisible, with the lighted part of the moon facing us.
Walter Freeman is an Assistant Professor in the Physics Department at the College of Arts and Sciences at Syracuse University. He says astronomical events like the black moon grab people’s attention because it’s kind of a bridge between two of humanity’s greatest achievements: fiction and mythology, and science.
Professor Freeman answers five questions about the July 31 “black moon”:
What is a “dark moon” and can you explain the science behind it?
“A ‘black moon’ is just a second new moon that occurs in a calendar month. It is no different from any other new moon from a science standpoint. When the Moon is on the same side of the Earth as the Sun, then the face of the Moon that receives the sunlight is directed away from us, and we cannot see it; the face of the Moon which is pointed at us is in the shadows, so we cannot see it. It’s a “new moon,” and it happens once every 29.5 days. Our months are a bit longer than that, so sometimes we get two per calendar month. This happens because the designers of the Gregorian calendar we use wanted each year to be the same length, and “stretched” the months to be a little longer than 29.5 days so that twelve months is exactly one year. . So if a new moon occurs around the start of a calendar month, the next one will occur before it is over. There is no science here; it’s just an artifact of how we keep time. “
How is it different from other lunar events, like a super wolf blood moon?
“A ‘blood moon’ is actually something astronomically interesting: a lunar eclipse, where the Earth’s shadow falls on the Moon. The term “blood moon” comes from the fact that red light from the Sun leaks around the edges of the Earth even during a lunar eclipse. “
How do you see it?
“There is nothing to see! A new moon is the phase of the Moon where it is invisible since the illuminated part is turned towards us.
“However, the fact that you cannot see the Moon allows you to see the stars, planets and the Milky Way better. When the Moon is above the horizon, the moonlight drowns everything except the brightest stars and planets. When the Moon is close to the news, it is also mostly found below the horizon at night. So this time of the month, when the Moon is almost new, is a great time to go and observe all the other things in the night sky.
“In particular, we have a very good view of Jupiter right now in the southern sky, visible after dark. If you look at Jupiter with binoculars, a small telescope, or a large telephoto lens, you can see Jupiter’s four largest moons; these are the same ones that Galileo saw four hundred years ago that convinced him that the Sun, and not the Earth, was at the center of the solar system.
Is this type of lunar event rare?
“A new moon happens every 29.5 days, so – astronomically speaking – it’s a pretty common event! Two new moons will occur in a month whenever the first new moon falls on the first day and a half of a 31-day month; this will happen every year or two. “
Why do you think these types of lunar events are attracting the attention of so many people?
“Astronomical events grab people’s attention because we’ve always told stories and created myths about things in the night sky. The bright red planet is named after the blood red Mars, the god of war. We named the beautiful planet visible morning and evening after Venus, the Roman goddess of beauty. Now we know more, of course: Mars is reddish because its surface contains rust, and Venus is in fact a sulphurous hell hot enough to melt lead thanks to an uncontrollable greenhouse effect. But knowing what planets and stars really are, and how they work, doesn’t mean we can’t think about myths too.
“So I think astronomy captures people’s attention because it’s kind of a bridge between two of mankind’s greatest achievements: fiction and mythology, and science.”