Mount Lemmon spies a comet in the south

A new comet, fairly bright though probably not for the naked eye, is sloping up.

This space view is from 20° north of the ecliptic plane and from longitude 235° (angles chosen after many tries!). Grid lines on the plane are 1 AU (astronomical unit, Sun-Earth distance) apart. The ram-horns symbol marks the vernal equinox direction. The Earth is exaggerated 500 times in size, the Sun 5 times. Stalks connect the comet to the plane at the beginning of each month of 2023.

Comet C/2021 T4 Lemmon was discovered on 2021 Oct. 7 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. (Another Comet Lemmon was C/2011 F6, which passed through perihelion in 2013.)

Mount Lemmon is the summit 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 BC.) 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 (as told in the cover picture story for Astronomical Calendar 2023).

But actually comet C/2021 T4, because of the geometry of its orbit, appeared quite northerly, at declination +12°, when it was found.

It is of the long-period kind; if it ever previously dropped from its remote home, 44,000 AU out, to the inner solar system, this will have been millions of years ago. During its present passage it will feel gravitational “perturbations” from the planets that will shorten its period to merely thousands of years.

Its orbit is inclined only about 20° to the ecliptic plane but in the retrograde direction – opposite to the direction in which the planets revolve. The result is that it will make a very long rapid sweep across our southern sky.

See the end note about enlarging illustrations.

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, and still at a dim magnitude of perhaps 10. On June 27 its distance from us will shrink to 1 AU.

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

In the next months it will climb north, becoming lower in the evening sky and more distant, and dimming by perhaps two or three magnitudes. It will be at perihelion, 1.48 AU from the Sun, on July 31; will ascend across the ecliptic on Sep. 10; and be at conjunction behind and north of the Sun on Nov. 9.

We always have to remember that comets’ brightness, and the size of their tails, can only be unreliably predicted, because they depend on the melting of ice and release of dust in these lumpy spinning objects.

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

__________

This weblog maintains its right to be about astronomy or anything under the sun.

ILLUSTRATIONS in these posts are made with precision but have to be inserted in another format.  You may be able to enlarge them on your monitor.  One way: right-click, and choose “View image” or “Open image in new tab”, then enlarge.  Or choose “Copy image”, then put it on your desktop, then open it.  On an iPad or phone, use the finger gesture that enlarges (spreading with two fingers, or tapping and dragging with three fingers).  Other methods have been suggested, such as dragging the image to the desktop and opening it in other ways.

Sometimes I make improvements or corrections to a post after publishing  it.  If you click on the title, rather than on ‘Read more’, I think you are sure to see the latest version.  Or you can click ‘Refresh’ to get the latest version.