Renowned British astronomer Guy Ottewell originally published this piece about Comet C/2021 T4 Lemmon on May 25, 2023. Reprinted with permission. Edits by EarthSky.

Comet C/2021 T4 Lemmon

Comet C/2021 T4 Lemmon was discovered on October 7, 2021, on images taken at the Mount Lemmon Observatory, northeast of Tucson in Arizona. T4 means the 4th discovery or recovery in the first half of October.

Mount Lemmon is the highest point of the Santa Catalina Mountains, one of four mountain ranges around Tucson. It’s not to be confused with Catilina, the conspirator who tried to seize power over the Roman republic in 63 BCE. I’m reminded of my speculation that the Navajos may have seen Canopus, the great star of the south, from one of the four sacred peaks surrounding their land. In fact, it’s shown as the cover picture for the Astronomical Calendar 2023.

When discovered, comet C/2021 T4, because of the geometry of its orbit, appeared quite northerly, at declination +12°.

Comet C/2021 T4 Lemmon is a long period comet

In fact, it’s a long-period comet; if it ever previously dropped from its remote home – at 44,000 AU out – to the inner solar system, it would have been millions of years ago. So during its present passage, it’ll feel gravitational perturbations from the planets that will shorten its period to merely thousands of years.

Its orbit is inclined about 20° to the ecliptic plane. However, it’s going in a retrograde direction, or opposite to the direction in which the planets revolve. The result is that it’ll make a very long rapid sweep across our southern sky.

Finder chart

At present the comet is 60° out in the morning sky, southerly (at declination -13°), 1.75 AU from the sun and 2 AU from Earth. However, it’s still at a dim magnitude of about 11. Then, on June 27, 2023, its distance from us will shrink to 1 AU.

On July 18, 2023, we will pass it at opposition. And around this time, it’ll be nearest to us, 0.54 AU, and brightest, perhaps about magnitude 8 or 7 but still below the unaided-eye limit. Its nearness will make it appear even farther south, at declination -56° on July 20.

Then in the following months it will climb north, becoming lower in the evening sky and more distant. At the same time it’ll be dimming by perhaps 2 or 3 magnitudes. It will reach perihelion, 1.48 AU from the sun, on July 31, 2023. Finally, it’ll ascend across the ecliptic on September 10, 2023, and be at conjunction behind and north of the sun on November 9, 2023.

Of course, we must remember that predictions of a comet’s brightness, and the size of their tails, can be unreliable. That’s because they depend on the melting of ice and release of dust in these lumpy spinning objects.

Comet-Hale Bopp still observable? Wow!

By the way, Alan Hale alerted us (Guy Ottewell) to this comet with a Facebook post on May 22. Alan was discoverer of the great comet Hale-Bopp (C/1995 O1). And, despite now being more than 47 AU away, it’s the first on the Minor Planet Center’s list of currently observable comets, not because of its present magnitude (about 20) but because it is the earliest-numbered non-periodic comet still considered observable at all.

Bottom line: Comet C/2021 T4 Lemmon was discovered from Mount Lemmon Observatory in 2021. It’s currently sweeping through the southern skies.

A new, close supernova

A new supernova in the Pinwheel Galaxy, aka M101, is the closest to Earth in a decade. Amateur astronomer Koichi Itagaki discovered it on May 19, 2023. The supernova should continue to brighten for a few days. It should remain visible to amateur astronomers with backyard telescopes for a few months. The supernova – named 2023ixf – lies in the direction of the constellation Ursa Major, near the end of the handle of the Big Dipper.

The last supernova in M101 was in 2011. Andy Howell, an astronomer at the University of California, Santa Barbara, explained on Twitter that the 2011 explosion resulted from a white dwarf that underwent a thermonuclear supernova. Andy said that the new supernova is most likely from a different cause, from a core collapse of a massive star at the end of its life. While observers won’t be able to see this supernova with the unaided eye, amateurs should be able to catch it backyard telescopes. As Andy said:

… this new supernova will increase in brightness over the coming days. You should be able to see it with backyard telescopes, for a few months, though it will just be a point of light.

How close is the closest in a decade?

M101 is 21 million light-years away. So even though it just appeared to us on Earth Friday, it occurred 21 million years ago. And even though it’s the closest in a decade, it’s still quite far away. In fact, for a supernova to have an effect on Earth, it would have to be within 50 light-years of our planet. You can rest assured that the new supernova won’t harm Earth. Instead, we get to watch a relatively rare event as scientists gather information on the new supernova.

Supernova before-and-after image

Look in the upper arm of this galaxy- you’ll see a star appear to blink in and out of existence. That’s a supernova! Very recently discovered in m101: the Pinwheel galaxy (which I happened to be shooting when this happened) pic.twitter.com/8hvplfXNtd

— Andrew McCarthy (@AJamesMcCarthy) May 20, 2023

Breaking news, today a new Type 2 Supernova appeared in the Pinwheel galaxy (M101) around lunchtime today and I got images tonight. This is a VERY rare event! Despite heavy forest fire smoke, light pollution and aurora, it still shines through strongly. Seems to have… pic.twitter.com/Jpmp2ZPjva

— ChasinSpin (@ChasinSpin) May 20, 2023

Supernova photo gallery

For the supernova images, each photographer chooses their own orientation. Therefore, you can see the supernova in various locations, such as in the 2, 11, 5, or 8 o’clock positions, for example. That’s because there is no standard way to capture a galaxy (there’s no “up” in space).

Pre-discovery images

Did you capture an image of the supernova you’d like to share with us? You can submit it to EarthSky Community Photos.

Bottom line: An amateur astronomer discovered a new supernova in M101 – the Pinwheel Galaxy – on May 19, 2023. The supernova will continue to brighten for a couple days and be visible to amateurs in backyard telescopes for a few months.

NASA posted this original article on May 16, 2023. Edits by EarthSky.

Help spot asteroids!

NASA’s new Daily Minor Planet project seeks your help discovering and tracking asteroids in a dazzling new data set. Remember asteroids, those lumps of rock tumbling through space left over from the formation of our solar system? There are so many reasons to find these objects. Some asteroids pose an impact hazard to Earth, while others are essential for humanity’s endeavor to explore, live, and work in space. Now there’s a new way you can help.

The Daily Minor Planet project uses data from the NASA-funded, University of Arizona-based Catalina Sky Survey, which collects more than 1,000 images per night. Carson Fuls, a science engineering specialist for the Catalina Sky Survey who heads the project, said:

We take so many images of the sky each night that we cannot possibly look through all of our potential real asteroids.

How to help

At the Daily Minor Planet, you’ll decide if the specks of light in the images look like genuine celestial bodies. Or, they might be false detections resulting from inconveniently timed “twinkles” of the star-studded background, dust on the telescope mirror, or other causes. After answering by clicking a yes or no button, you can either write a comment or move on to the next set of images.

The new Daily Minor Planet project replaces the Catalina Outer Solar System Survey project, which is now complete. If you contributed to the Catalina Outer Solar System Survey project, thank you! The science team learned from their experience working with you on that project and cleared up some bottlenecks in their data pipeline. The new Daily Minor Planet will feature new images uploaded daily. Come give it a try!

Fuls said:

I thought it would be great if people could do what we do every night. We see this website throwing open the doors: Do you want to look for asteroids, too? If so, come on in.

But wait, there’s more!

Can’t get enough asteroids? You’ll also enjoy searching for comet-like objects hiding in the asteroid belt with the Active Asteroids project. Or, get a group together to join the International Astronomical Search Collaboration (IASC).

Bottom line: NASA’s new project, the Daily Minor Planet, is asking for you to help spot asteroids. You’ll help decide if the specks of light are asteroids or other objects such as stars or telescope dust.

Via NASA

Read more: Detecting asteroids near the sun with NEOMIR

Use Vega to locate the Keystone in Hercules

In late spring, from mid-northern latitudes, you can easily find the brilliant star Vega in the eastern sky at dusk and nightfall. The brilliant blue-white star Vega acts as your guide star to the Keystone, a wedge-shaped pattern of four stars in the constellation Hercules.

Look for the Keystone asterism – star pattern – to the upper right of Vega. Or hold your fist at arm’s length, it’ll easily fit between Vega and the Keystone.

Also, you can locate the Keystone by using Vega in conjunction with the brilliant yellow-orange star Arcturus. The Keystone is found about 1/3 of the way from Vega to Arcturus, the two brightest stars to grace the Northern Hemisphere’s spring and summertime sky. From mid-northern latitudes this time of year, Arcturus is found quite high in the eastern sky at nightfall and evening. Then, by late evening, Arcturus will have moved high overhead.

Use the Keystone to find M13

Furthermore, the Keystone is your ticket to find a famous globular star cluster in Hercules, otherwise known as the Hercules cluster, aka Messier 13 or M13.

Most likely, you’ll need binoculars to see the Hercules cluster. Although sharp-eyed people can see it with the unaided eye in a dark, transparent sky. But through binoculars, this cluster looks like a dim smudge or a somewhat fuzzy star. However, a telescope begins to resolve this faint fuzzy object into what it really is, a great big, globe-shaped stellar city populated with hundreds of thousands of stars!

Then, later in the evening, the Keystone and the Hercules cluster swing high overhead after midnight, and are found in the western sky before dawn.

Photos of M13 from EarthSky Community Photos

Bottom line: Let the bright star Vega guide you to a famous star pattern in Hercules – called the Keystone – and then to the Hercules cluster, aka M13, a famous globular star cluster.

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May 2023 meteors … the Eta Aquariids

Moonlight will obscure the 2023 Eta Aquariids.

When to watch: Full moon falls near the peak of the 2023 Eta Aquariid shower. If you want to try watching in moonlight, try the mornings of May 5, 6 and 7, 2023, in the hours before dawn. Why before dawn? See Radiant chart above. The American Meteor Society is listing 15 UTC on May 6 as the shower’s predicted** peak time. But times vary between different experts. And the peak of this shower stretches out over several days. So you can expected elevated numbers of meteors a few days before and after the peak time … albeit in moonlight.
Nearest moon phase: In 2023, full moon will fall at 17:34 UTC on May 5. Moonlight will obscure the 2023 Eta Aquariids.
Radiant: Rises in the wee hours, climbing toward its highest point at dawn. That’s why before dawn is the best time to watch this shower.
Duration of shower: April 15 to May 27.
Expected meteors at peak, under ideal conditions: In the southern half of the U.S., you might see 10 to 20 meteors per hour under a dark sky, with no moon, when the radiant is high in the sky. Farther south – at latitudes in the Southern Hemisphere – you might see two to three times that number.
Note: The Eta Aquariids’ radiant is on the ecliptic, which rides low in the sky on spring mornings as seen from the Northern Hemisphere. That’s why this shower favors the Southern Hemisphere. It’s often that hemisphere’s best meteor shower of the year … but not in 2023, when moonlight will drown out most meteors.

Visit EarthSky’s meteor shower guide for 2023

Report a fireball (very bright meteor) to the American Meteor Society: it’s fun and easy!

The Eta Aquariids’ parent comet

This section is by the late, great Don Machholz (1952-2022), who discovered 12 comets …

The object responsible for the Eta Aquariid meteor shower – that is, its parent comet – is the famous Halley’s Comet. This comet is in a retrograde orbit around the sun. That means it runs around the sun in the opposite direction from Earth and all the other planets. As a result, we pass near its path twice, one time along the outbound portion of the comet’s orbit. That happens every early May, causing the Eta Aquariid meteor shower. The other time is along the inbound portion of the comet’s orbit, and that passage causes the Orionid meteor shower in late October of each year.

Halley’s Comet orbits the sun on an average of every 76 years (the range is from 74 through 79 years due to perturbations of the planets). So, in most years, the comet is nowhere near when we sweep through its orbit, and when debris left behind by the comet enters our atmosphere to create Halley’s two meteor showers.

Perhaps you saw Halley’s Comet when it returned last, in 1985/86. It has been observed since the year 240 BCE. Halley’s Comet will be back in 2061. Presently the comet is traveling away from the sun at about 0.6 miles a second (0.9 km/sec). In the year 2023, Halley’s Comet is beyond the orbit of Pluto.

In November or December of 2023, the comet will reach its farthest point from the sun that binds it in orbit. Then – pulled inexorably by the sun’s gravity – it will curve around and head back toward the inner solar system again.

While waiting for Halley’s Comet to return, watch for the next best thing: the Eta Aquariid meteor shower in early May.

More about this shower’s radiant

If you trace the paths of the Eta Aquariid meteors backward, they all seem to radiate from a certain point in front of the constellation Aquarius the Water Bearer. This point on the sky’s dome is called the radiant of the meteor shower, which nearly aligns with the faint star Eta Aquarii. Hence, this meteor shower gets its name from this star.

Eta Aquarii is one of the four stars making up the Y-shaped Water Jar asterism in the northern part of Aquarius. If you can find the Water Jar in the constellation Aquarius, you’ve as good as located the radiant point for the Eta Aquariid meteors. The alignment of the radiant and the star is, of course, coincidental. Eta Aquarii is some 170 light-years away – trillions upon trillions of miles away – while the Eta Aquariid meteors burn up nearby – only 60 miles (100 km) above Earth’s surface.

Meteor shower radiants are sometimes misunderstood by casual meteor-watchers. You don’t need to know where they are to watch a meteor shower. That’s because the meteors fly every which way across the sky, in front of numerous constellations. However, the higher a shower’s radiant appears in your sky, the more meteors you’re likely to see. For the Eta Aquariids, the radiant soars highest in the nighttime sky just before dawn. That’s one of the reasons why you can expect to see the most meteors in the wee morning hours.

How to view a meteor shower

As with all meteors in annual showers, no special equipment to watch the Eta Aquariids. But a little luck always helps.

Find a dark, open sky away from artificial lights, and sprawl out on a reclining lawn chair.

Make yourself comfortable with a hot flask of you favorite beverage. Keep warm but not so snug that you fall asleep!

Meteor watching is a lot like fishing. Sometimes you catch a good number of them, and sometimes you don’t.

Eta Aquariid meteor shower photos from EarthSky’s community

Bottom line: May’s Eta Aquariid meteor shower has a broad peak and often can be watched over several mornings. But, in 2023, moonlight interferes.

**Predicted peak times and dates for meteor showers are from the American Meteor Society. Note that meteor shower peak times can vary. Back to top.

Read more: Why the Eta Aquariids are best from the Southern Hemisphere.

Meteor showers: Tips for watching the show