What's Up for September? Following the crescent Moon, the September equinox and — wait — where did Mars go?
We're in a several-month period right now when the new moon falls right around the end of each month. This means we get to enjoy lovely waning crescent moons at dusk for the first few days of each month, and delightful waxing crescents in the predawn sky near the end of each month.
This month, look low in the west about half an hour after sunset to enjoy the crescent moon on September 1st through the 4th, with the Moon appearing a bit higher in the sky each night. By the 5th, the first-quarter (that is, half-full) Moon winds up here, just a couple of degrees to the right of Jupiter.
At the end of the month, from September 23rd to the 27th, look east half an hour before dawn for an increasingly slimmer crescent, that appears lower in the sky each day.
As you make your lunar observations, remember that for many thousands of years, the cycles of the Moon and Sun were the basis of human timekeeping. And many traditional cultures still rely on these cycles to mark special events.
A few months ago, it seemed like the Red Planet, Mars, was a constant companion in the evening sky. But as our two planets moved along in their orbits this summer, Mars has drifted further into the glare of the Sun, finally disappearing from our skies altogether in July. (Did you notice?)
In late August and early September this year, Mars is more or less behind the Sun as seen from Earth. This has implications for spacecraft at Mars, like NASA's Insight lander and Curiosity rover.
This event, called solar conjunction, happens about every two years. During this time, mission controllers on Earth stop sending commands to our spacecraft, in order to avoid potential radio interference from the Sun. A few weeks later, when Mars has moved farther apart from the Sun as seen from Earth, normal communications can resume.
For those of us eager for a peek at Mars with our own eyes once again, it'll return to our pre-dawn skies in early November.
September 23rd marks the equinox, with day and night being of equal length. This marks the beginning of fall in the Northern Hemisphere. And although this means it's time to bid farewell to those long summer days, the upshot for stargazers is longer nights, meaning more time to look up!
Here are the phases of the Moon for September.
You can catch up on all of NASA's current and future missions at nasa.gov.
I'm Preston Dyches from NASA's Jet Propulsion Laboratory, and that's What's Up for this month.
Correction: The video states that the Moon will be waning at the beginning of the month and waxing at the end. In fact it is the opposite — the Moon is waxing at the beginning, and waning at the end. We regret the error.
About Mars Solar Conjunction
From Earth's point of view, in late August and early September, Mars is more or less behind the Sun. The Sun expels hot, ionized gas from its corona, which extends far into space. During solar conjunction, this gas can interfere with radio signals when engineers try to communicate with spacecraft at Mars, corrupting commands and resulting in unexpected behavior from our deep space explorers. To be safe, engineers hold off on sending commands when Mars disappears far enough behind the Sun's corona that there's increased risk of radio interference.
More about Mars solar conjunction:
About the Equinox
On the September equinox, day and night are of equal length. From there until the winter solstice in December, nights become longer than days. This reduction in the number of hours of sunlight per day is what leads to seasonal cooling we associate with fall and winter. Of course, it's the opposite story in the Southern Hemisphere, where the September equinox ushers in the beginning of summer.
Other planets with seasons, like Saturn, also experience equinoxes. NASA's Cassini spacecraft observed the most recent Saturn equinox in 2009: https://solarsystem.nasa.gov/resources/10695/equinox-at-saturn/
More about equinoxes:
Additional astronomy & skywatching info from NASA's Night Sky Network: https://nightsky.jpl.nasa.gov
Meteor stories from NASA's Watch the Skies Blog: https://blogs.nasa.gov/Watch_the_Skies/tag/meteor/