A Fine Southern Apparition for Comet T4 Lemmon

The time to catch Comet T4 Lemmon is now, before it vanishes for another 36,000 years.

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

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

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 inclination orbit, moving backwards or retrograde versus 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 a 1.000034 eccentricity inbound and a period most likely measured in millions of years, the comet’s orbit was altered into a 36,000-year return period outbound, due to a 0.869 AU encounter with Jupiter during the current passage.

The 2023 apparition for Comet T4 Lemmon starts the end of July, in the southern constellation of Ara the Altar. Currently shining at +8th magnitude, we’ve already seen some fine shots of the comet taken from the southern hemisphere in June and July 2023. We’re fortunate that the comet is visiting the inner solar system when it is, as it just passed 0.54 AU (81 million kilometers) from the Earth on July 21st. Comet T4 Lemmon crossed opposition on July 18th moving from the morning to the evening sky, and bottomed out on July 20th at a southern declination of -56 degrees. The comet is currently moving at three degrees per day across the sky (about the diameter of six Full Moons a day) as it slowly recedes from the Earth.

Comet T4 Lemmon from June 30th, as it threads between the wide double stars of Kappa Sculptoris. Credit: Pepe Chambó/Cometografia.

Comet T4 Lemmon is currently a bashful one for northern hemisphere observers, but don’t despair; the view improves going in to August as the comet creeps up towards the ecliptic. For example, Tampa, Florida sees the comet low to the south (20 degrees above the southern horizon) after dusk on August 1st.

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

T4 Lemmon
Comet T4 Lemmon crosses the field of view of the supernova-hunting Zwicky Transient Facility. Credit: ZTF.

Here’s quick rundown on celestial dates with destiny for Comet T4 Lemmon over the next few months. Unless otherwise noted, ‘near’ for the following list means less than a degree away:


25- Passes two degrees from the +5.7 magnitude globular cluster NGC 6397.

26- Passes near +2.8 magnitude star Alpha Arae.

29-Crosses the galactic plane northward, into the constellation Scorpius.

30-Crosses into the constellation Norma

31-Reaches perihelion, at 1.48 AU from the Sun.

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


1-Crosses back into Scorpius.

3-Crosses into Lupus.

4-Passes near the +3.4 magnitude star Eta Lupi.

11-Crosses into the constellation Libra.

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

23-Passes within two degrees from the 45% illuminated waxing crescent Moon.

Light Curve
The projected and observed light curve for Comet T4 Lemmon. Credit: Weekly Information About Bright Comets.


1-May drop back down below +10th magnitude.

11-Crosses the ecliptic plane northward.

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

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

I would approach observing and imaging Comet T4 Lemmon by treating it as a fuzzy +8th magnitude globular cluster, that stubbornly refuses to resolve in focus: simply sweep the suspect field with a wide-field telescopic view or (or fave) binoculars until the hirsute-looking ice-ball pops into view.

Be sure to track down Comet T4 Lemmon, coming to a sky near you this summer; humanity won’t catch sight of it again until well into the 38th millennium AD.

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