August 2021 – Watch the Skies

This month we’ll get to see a Full Moon on Aug. 22, 2021, known by some early Native American tribes of the northeastern United States, as the Sturgeon Moon. The name was given to the Moon because the large sturgeon fish of the Great Lakes, and other major lakes, were more easily caught at this time of year. But that’s not all! We also get to see a Blue Moon!

We’ve all heard the phrase “once in a Blue Moon,” which usually refers to something that rarely happens. Blue Moons do sometimes happen in Earth’s night sky, giving rise to this phrase. But what is a Blue Moon?

One way to make a Blue Moon is by using a blue filter. Credit: NASA

Well, we have two kinds of Blue Moons – monthly and seasonal.

A monthly Blue Moon is the second Full Moon in a calendar month with two Full Moons. Then, there’s a seasonal Blue Moon – the third Full Moon of an astronomical season that has four Full Moons.

In astronomy, a season is the period of time between a solstice and equinox, or vice versa. Each season – winter, spring, summer or fall – lasts three months and usually has three Full Moons, occurring about 30 days apart. Because June’s Full Moon came just a few days after the June (Summer) solstice, we will see four Full Moons in the current summer season, which ends at the September equinox on Sept. 22.

The third Full Moon – our seasonal Blue Moon – will happen on Aug. 22. All Full Moons are opposite the Sun, as viewed from Earth, rising fully illuminated at local time around sunset and setting around sunrise.

Perhaps you’re wondering if the Moon ever actually takes on a blue color. Well, Blue Moons that are blue in color are extremely rare and have nothing to do with the calendar or the Moon’s phases; they don’t have to be Full Moons either. When a blue-colored Moon happens, the blue color is the result of water droplets in the air, certain types of clouds, or particles thrown into the atmosphere by natural catastrophes, such as volcanic ash and smoke. Also, blue-colored Moons in photos are made using special blue filters for cameras or in post-processing software.

In 1883, an Indonesian volcano called Krakatoa produced an eruption so large that scientists compared it to a 100-megaton nuclear bomb. Ash from the Krakatoa explosion rose as high into the atmosphere as 80 kilometers (50 miles). Many of these ash particles can be about 1 micron in size, which could scatter red light and act as a blue filter, resulting in the Moon appearing blue.

Blue-colored Moons appeared for years following the 1883 eruption. Many other volcanoes throughout history, and even wildfires, have been known to affect the color of the Moon. As a rule of thumb, to create a bluish Moon, dust or ash particles must be larger than about 0.6 micron, which scatters the red light and allows the blue light to pass through freely. Having said all of that, what we call a Blue Moon typically appears pale grey, white or a yellowish color – just like the Moon on any other night.

Generally, Blue Moons occur every 2 to 3 years. Our last Blue Moon was on Oct. 31, 2020 – the night of Halloween. Mars was red and very large, since it was closer to Earth, and it was seen in the sky near the Blue Moon. Coincidently, this year’s Blue Moon will appear near planets again, but this time Jupiter and Saturn! We won’t see another Blue Moon until August 2023.

Learn more about Earth’s Moon here.

by Lance D. Davis

By Bill Cooke, NASA Meteoroid Environments Office

Many Perseid-related news stories and social media posts state that the maximum rate is about 100 meteors per hour, which is a lot. So, folks get excited and go out on the peak night, braving mosquitos and other nightly hazards. But they are often disappointed; we routinely hear, “I went out and only saw a few meteors. Not even 20, much less 100!” And they would be right. The problem is that the 100 per hour is a theoretical number used by meteor scientists and does not convey what people are actually going to see.

In the 1980’s, meteor researchers were searching for a way to compare the meteor shower rates observed by various individuals and groups across the globe. People were reporting the rates, but the differences in sky conditions, radiant altitude and observer eyesight made getting a comprehensive view of shower activity difficult.

So, the meteor researchers put their heads together and came up with the concept of a ZHR, or Zenithal Hourly Rate. The ZHR is what you get after you correct the observed rates for the sky conditions, the altitude of the radiant above the horizon and observer biases. In other words, it is basically what a perfect observer would see under perfect skies with the meteor shower radiant straight overhead – which never happens!

The often-quoted ZHRs overestimate the meteor rates people actually see – sometimes by a lot. Fortunately, we can take the ZHR and invert things to get the hourly rates for certain locations and circumstances – it’s only math, after all. We have done this for select locations in the United States, producing the following maps.

These maps show the hourly rates that can be expected on the night of the Perseid shower’s peak, provided there are no clouds in the sky. (It’s hard to account for partial cloud cover.)

These rates assume you are out in the country, where lots of stars and the Milky Way are visible and no clouds, of course:

So, instead of 100 Perseids per hour, people in the U.S. can reasonably expect to see around 40-ish Perseids in the hour just before dawn on the peak nights. That’s about one every couple of minutes – not bad. However, we are assuming you are out in the country, well away from cities and suburbs.

What rates can you expect if you want to do your Perseid watching from the neighborhood? We also computed that:

The brighter skies of the suburbs greatly cut down the rates. We have gone from a Perseid every couple of minutes to one every 6-7 minutes – a factor of three reduction. This explains the great disappointment expressed by many casual Perseid watchers; they go outside, expecting to see at least a meteor a minute and end up with 10 or less in an hour. The brightness of your sky is everything in meteor observing – you have to get away from the lights!

But what about those in cities? The rates are close to zero:

Ugh! City dwellers might see a Perseid or two in an hour. Not very inspiring. Perhaps the only good news is that, if someone in a city sees a Perseid, it has to be really, really bright and spectacular.

Want to see Perseids? Then head out into the dark – it’s worth it!

Check out our previous blog post, The Perseids are on the Rise, for more information on the Perseids and tips on how to observe them.

Month: August 2021

August 2021 Brings Rare Seasonal ‘Blue Moon’

How many Perseids will I see in 2021?