See Neptune at its best and more top stargazing in September

Opposite the sun this month, the eighth planet in the solar system is at its most visible, while Mars disappears and its rovers go quiet.

As nights begin to lengthen across the northern hemisphere, skywatchers are able to spend more time under the stars while temperatures are still pleasant. From ghostly celestial glows to neighboring worlds, an amazing cosmic treasure trove awaits.

Mark your September calendar and gaze skyward on the next clear night!

Mars solar conjunction—September 2

Mars is on the exact opposite side of the sun from Earth today. The planet is not only invisible to skywatchers but also to NASA’s Mars landers, rovers, and orbiters too. Every two years solar conjunction occurs between our two worlds, and NASA engineers put a halt to sending all commands to the Martian spacecrafts out of concern the signals may become corrupted. According to the space agency, out of an abundance of caution, between August 28 and September 7, Curiosity stops driving, the Insight lander parks its robotic arm, and Mars orbiters temporarily pause relaying data to Earth.

Jovian triangle—September 5

Look towards the southern sky for an eye-catching celestial triangle this evening, formed by the first-quarter moon, Jupiter, and the 550 light-year distant star Antares. All three objects are hanging out around the Scorpius constellation region.

Antares is a stellar goliath to reckon with. A red supergiant star, it’s 440 times larger than our sun. If we were to replace our sun at the center of our solar system with Antares, it would easily engulf all the inner planets out to Mars. In fact it’s such a cosmic heavyweight, astronomers estimate that it may eventually go supernova.

Moon and Saturn—September 7

Look towards the southern sky at dusk for the waxing gibbous moon paying a visit with Saturn.

Look carefully below the cosmic pair for the bright asterism known as the Cosmic Teapot. This distinctive pattern of stars, nestled within the constellation Sagittarius, marks the area in Earth’s sky where the core of Milky Way galaxy is located.

Neptune at its best—September 10

The eighth and last major planet in our solar system reaches opposition. This evening Neptune is exactly opposite from the sun in our skies, which for skywatchers means that this icy blue giant is at its biggest and brightest. It shines at only a feeble 7.8 magnitude, so you will still need a small telescope to hunt it down among the backdrop of faint stars of the constellation Aquarius in the low southern sky.

However, we have a convenient guidepost to finding Neptune in the early morning hours of September 6. Look for the planet parked next to the faint, 4.2 magnitude, naked-eye star Phii Aquarii.

Harvest moon—September 13

Watch the near full moon sail up the eastern horizon soon after sunset. It reaches official full status at 12:32 a.m. EDT on Saturday, September 14, as it glides into the southeast sky.

The full moon nearest the fall equinox is known as the harvest moon, a name probably coined by farmers in the northern hemisphere since its added light is said to have helped them gather in their crops.

Binoculars will easily show off the moon’s dark patches visible with the naked eye. Called plains or maria in Latin, meaning seas—these are vast, ancient lava plains formed billions of years ago when magma from the moon’s interior spilled out onto the surface, triggered by giant asteroid impacts.

With telescopes, the views get even more exciting—you can get sharp views of hundreds of ridges, mountains, cliffs, and craters up close.

September equinox—September 23

The equinox occurs at 3:50 a.m. ET, officially marking the time of year we kick off the fall season in the northern hemisphere and the start of spring in the southern hemisphere. The word equinox comes from Latin meaning “equal night” and refers to the 12-hour long day and night that occur only on this particular day of the year.

Looking at the mid-day position of the sun over the summer season, northern hemisphere skywatchers will notice that it has been slowly sinking closer to the southern horizon, creating ever longer shadows.

It’s only on the spring and autumnal equinox that the sun rises due east and sets due west.

Astronomically speaking the September equinox marks one of the four major turning points in the cycle of seasons. The Earth spins on its axis, which is tilted at 23.5 degrees with respect to its orbital plane. On these days however the Earth’s axis is neither tilted away nor toward the sun, and both northern and southern hemispheres experience equal amounts of sunshine.

Moon with twins—September 23

This morning about 45 minutes before sunrise look toward the east as a pretty crescent moon poses with two bright stars, Castor and Pollux, the Gemini twins. Castor is a brilliant white star, while Pollux is an orange giant. They do not have any physical relation, and in fact 51 light-year distant Castor is actually part of a system of six stars that orbit each other. Backyard telescopes can easily separate Castor into a visual binary system. Both stars are separated from each other by as much space as the diameter of Pluto’s orbit and take 440 years to orbit one another.

Moon buzzes Beehive—September 24

At least an hour before local sunrise try using binoculars to spot the Beehive star cluster tucked underneath the thin crescent moon in the constellation Cancer, the crab.

Moon lies with Leo—September 26

Only a couple mornings after its dance with the celestial crab, the moon pairs up with Regulus, the 79 light-year star that marks the heart of Leo, the lion. This brilliant celestial jewel is the 21st brightest star in the entire sky and pins down a group of stars that mark out the head of the lion in the form of a backwards question mark.

Regulus is 3.5 times larger and 140 times more inherently bright than our sun. It spins on its axis so fast—making one full rotation in less than 16 hours—that it appears flattened at its poles.

Zodiacal light—September 26

About an hour before sunrise—and for the next two weeks afterward— keen skywatchers in the northern hemisphere can hunt down one of the most elusive of astronomical phenomena— Zodiacal Lights.

This pyramid-shaped beam of light is easily mistaken for the lights of a far-off city just over the dark horizon in the countryside. It has also been called the “false dawn.” But this light is more ethereal; it’s caused by sunlight reflecting off cosmic dust between the planets.

The best time to catch this ghostly sky light is about an hour before sunrise at morning twilight, looking toward the eastern horizon.

This celestial phenomenon is the reflection of billions of dust-size particles left behind in interplanetary space after the planets formed about 4.6 billion years ago.

Clear skies!

Andrew Fazekas, the Night Sky Guy, is the author of Star Trek: The Official Guide to Our Universe and the 2nd Edition of The Backyard Guide to the Night Sky. Follow him on Twitter, Facebook, Instagram, and his website.

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