Road Map of the Stars: October 2020 + Bonuses

Oct 11

A tour of our nighttime sky in October. Learn about how our ancestors used the stars long ago, the stories they told, and how you can use the stars today!

Supplementary Material

October 2020 Star Chart

Click the image for a printable PDF star chart of the October 2020 sky, which also lists astronomical events to look for this month!

Mass and Gravity

During the show, Mary asked if mass affects gravity. Yes — mass affects gravity, and I wanted to more thoroughly explain that here using models.

The more massive an object, the more gravitational “pull” it has. Everything that has mass in our universe has gravity. Even you! But the Sun is far more massive than everything else in the solar system combined. Because of that, its gravitational “pull” reaches through the solar system and causes the planets to orbit around it.

Now, though we say planets orbit the Sun, it's more accurate to say that the planets and Sun orbit each other. The planets also have mass, so they also have gravity! They are “pulling” on the Sun just like the Sun is pulling on them (but to a much lesser extent, because the planets are far less massive). In-fact, when two objects pull on each other, it looks as if they are both orbiting a central point— a mutual point of orbit — called the barycenter!

In these animations, the barycenter is represented by the red “+”. The barycenter is always closer to the more massive body. In the orbit of the Earth and Sun (the Earth-Sun System) , the barycenter is inside of the Sun, which is why the Sun barely appears to wobble!

For a more extreme case, let’s consider Pluto and its moon Charon.

Pluto and Charon are much closer in mass to each other than anything in our solar system is to the Sun. Because of this, the barycenter, in the Pluto-Charon System, is outside of Pluto! So the two appear to gracefully dance with one another while both obviously rotating around that barycenter!

*Note: These animations are not to scale.

More massive bodies, like Jupiter, have more gravity and thus, make the Sun’s “wobble” more apparent. Because Jupiter is far more massive than the Earth, it has a greater “pull” on our Sun! We can see now — in the Jupiter-Sun System animation — that the Barycenter is still inside the sun but is slightly closer to Jupiter. This causes the Sun to wobble slightly more!

Blue Moon and Full Moon Names

During the show, Mary asked for the definition of “Blue Moon”. Becca and I looked into this, and were surprised to find that the answer is not straightforward!

Today, “Blue Moon” can refer to the second full Moon to occur in a month, but—though the term “Blue Moon” has been around for over four-hundred-years— that definition was only popularized in the 1980s! It was adopted by pop culture when a radio show gave this definition, but the source the radio host used misinterpreted the definition from the Maine Farmers’ Almanac.

Have you ever played the game of "Telephone" — a game where a long line of people try to relay a sentence? When the sentence reaches the last person in line, it's often drastically different than the original sentence. That's what happened in this case! You can read about it here.

The definition of “Blue Moon” before that, applied to the third full Moon in a season that had four full moons. To elaborate, most years have twelve full Moons — three per season. How-ever, about seven times every nineteen years, there are thirteen full Moons in a year, meaning that one season has an extra full moon. But if the fourth moon is the “extra” one, why was it decided that the third (rather than the fourth) full moon in that season is called the “Blue Moon”? Because that made it possible for other full moons with established names (such as the Lenten Moon and Paschal Moon) to maintain their defined spots on the calendar in relation to the equinoxes and solstices.

Today, both these definitions of “Blue Moon” — the second full moon in a month or the third full moon in a season of four full moons — are accepted.

This astronomical definition of "Blue Moon" was used at least as far back as 1819, but in everyday conversation the definition of Blue Moon goes back even further. In the 16th century, the phrase "Blue Moon" was used to express something utterly impossible, as we'd use the phrase "When pigs fly" today, because the moon wasn't known to appear blue. However when lots of dust is in the sky, the moon can appear blue, and this was observed in 1883 due to the explosion of Krakatoa, in 1927 due to an extra long dry season in India, and in 1953 due to forest fires in Canada. Instances such as this changed the colloquial meaning of "Blue Moon" from something impossible to something rare.

Sources and Extra Reading

Why Does Venus Spin Backwards?

During the show, I discussed how Venus spins clockwise because it was flipped upside down during the early days of our solar system’s formation. But it may be hard to visualize why an object that is spinning counter-clockwise would be spinning clockwise if flipped upside down. This concept is easier to grasp if one has a fidget spinner to experiment with!

I’ve made an animation showing how when something is spun and then flipped over, it will appear to be spinning in the opposite direction. At the beginning of this video, the fidget spinner appears to be spinning clockwise. But, when flipped over, it appears to be spinning counter-clockwise. Its spinning motion wasn’t altered, but because we’re observing it from a different perspective, it now appears to spin clockwise.

This is likely what happened to Venus during the early formation of the solar system. When Venus formed, it was very likely rotating counter clock-wise, like most of the other planets in the solar system.

How-ever, one or more objects are thought to have collided with Venus and flipped it upside down, like this fidget spinner. And though Venus’s spinning motion continued in the same direction it was spinning before the collision, because it’s now upside down, that direction now appears to be clockwise from our perspective.

Additionally, you may notice that the fidget spinner began spinning more slowly after it was flipped. This is not an illusion. The act of flipping the axis of the fidget spinner “took” some of its rotational energy away, so it indeed began to spin more slowly! Like this fidget spinner, Venus lost rotational energy after being flipped. Consequently, it spins so slowly that it only rotates twice during its orbit around the sun, meaning one Venetian year (which is 225 Earth days) is only about 2 Venetian days!

Star Formation

I mentioned in the show that Albireo A and Albireo B probably formed from the same cloud of gas and dust. I wanted to elaborate on that: When incredibly massive stars (roughly four times more massive than our Sun) explode, what's left behind is a cloud of gas and dust. New, less-massive stars form when some of that gas and dust combines (due to gravity or other forces) and becomes so dense and hot that nuclear fusion begins. (Nuclear fusion is what our Sun and all other luminous stars do to create light and heat). A dust and gas cloud resulting from the explosion of a massive star can yield thousands of new, less-massive stars. Albireo A and B are close enough to one another that scientists think they may have formed from the same cloud of gas and dust, but they're far enough apart that it’s unlikely they’re orbiting each other.

James Sanders