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Menopausal Mother Nature

News about Climate Change and our Planet


Comet C/2021 T4 Lemmon is sweeping through southern skies

Comet C/2021 T4 Lemmon: Oblique view of Earth orbit and parabolic path of comet sweeping around it.
This space view is from 20° north of the ecliptic plane and from longitude 235°. Grid lines on the plane are 1 astronomical unit (AU), the distance between the sun and the Earth. The ram’s-horn symbol marks the vernal (March) equinox direction. The Earth is exaggerated 500 times in size, and the sun 5 times. Stalks connect the Comet C/2021 T4 Lemmon to the ecliptic plane at the beginning of each month of 2023. Image copyright Guy Ottewell. Used with permission.

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.

Constellation chart, with labeled stars and Milky Way, with curved line showing path of comet.
Chart showing the location of Comet C/2021 T4 Lemmon starting in July through November 2023. Chart copyright Guy Ottewell. Used with permission.

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.

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