A beautiful southern appearance for comet T4 Lemmon

THE it’s time to capture comet T4 Lemmon, before it fades away for another 36,000 years.

Often, icy intruders creep into the inner solar system, only to vanish once more into the abyss. Such is the case with long-period comet C/2021 T4 Lemmon, heading towards perihelion early next week.

The comet was discovered by the Catalina Sky Survey based on Mount Lemmon outside Tucson, Arizona on the night of October 7th, 2021, slowly moving through the constellation Taurus Taurus. Bright at magnitude +20 and 6.4 astronomical units (AU, 595 million miles or 958 million kilometers) distant beyond Jupiter’s orbit at the time of discovery, astronomers quickly realized that Comet T4 Lemmon could be a moderately bright binocular object in the summer of 2023.

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The orbit of comet T4 Lemmon, showing the comet near perihelion. Credit: NASA/JPL.

And here we are. The comet is also on a steep 160-degree orbit, moving backwards or retrograde relative to the planets in the inner solar system. Curiously, the comet may also be a first-time visitor from the Oort Cloud. On an orbit with an eccentricity of 1.000034 in and a period most likely measured in millions of years, the comet’s orbit was modified to a return period of 36,000 years out, due to a 0.869 AU encounter with Jupiter during the current pass.

The 2023 apparition for comet T4 Lemmon begins in late July, in the southern constellation of Ara the Altar. It currently shines at +8th magnitude, we’ve already seen some nice images of the comet taken from the Southern Hemisphere in June and July 2023. We were lucky that the comet is visiting the inner solar system when it is, as it just passed within 0.54 AU (81 million km) of Earth on July 21st. Comet T4 Lemmon crossed opposition on July 18th moving from the morning sky to the evening sky and bottoming out on July 20thth with a southern declination of -56 degrees. The comet is currently moving three degrees a day across the sky (about the diameter of six full moons a day) as it slowly moves away from Earth.

Comet T4 Lemmon on June 30, as it slips between the large double stars of Kappa Sculptoris. Credit: Pepe Chamb/Cometography.

Comet T4 Lemmon is currently shy of Northern Hemisphere observers, but don’t despair; the view improves going into August when the comet slowly approaches the ecliptic. For example, Tampa, Florida sees the comet in the low south (20 degrees above the southern horizon) after sunset on August 1.

Meanwhile, Cape Town in South Africa sees the comet at a generous 70 degrees above the southern horizon on the same date.

T4 lemon
Comet T4 Lemmon crosses the field of view of the Zwicky Transient Facility chasing a supernova. Credit: ZTF.

Here is a quick summary of the celestial dates with the fate of comet T4 Lemmon in the coming months. Unless otherwise noted, close for the following list means less than one degree away:


25- It passes two degrees from the globular cluster NGC 6397 of magnitude +5.7.

26- Pass near the +2.8 magnitude star Alpha Arae.

29-Cross the galactic plane to the north, in the constellation Scorpius.

30-Cross the constellation of Norma

31-Reaches perihelion, 1.48 AU from the Sun.

The celestial path of comet T4 Lemmon through August. Credit: Starry Night.


1-Go back to Scorpius.

3-Cross in Lupus.

4- Pass near the star of magnitude +3.4 Eta Lupi.

11-Cross the constellation Libra.

17-Pass near the +3.3 magnitude star (Brachium) Sigma Librae.

23-Pass within two degrees of the crescent Moon illuminated at 45%.

Light curve
The projected and observed light curve for comet T4 Lemmon. Credit: Weekly information on bright comets.


On May 1, it drops below +10th size.

11-Cross the plane of the ecliptic to the north.

12 – Passes just 8 from the star of magnitude +2.8 Zubenelgenubi (Alpha Librae).

After mid-September, the comet spends the remainder of 2023 heading outward across the solar system through Libra, crossing solar conjunction seven degrees from the Sun on Nov. 9th.

I would approach observing and imaging Comet T4 Lemmon by treating it as a fuzzy +8th magnitude globular cluster, stubbornly refusing to focus: just sweep the suspect field with a wide-field or telescopic sight (or binoculars of your choice) until the shaggy-looking iceball comes into view.

Be sure to track down Comet T4 Lemmon, which will arrive in a sky near you this summer; humanity will not see him again until well into 38 yearsth millennium AD

#beautiful #southern #appearance #comet #Lemmon
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