The Astronomer

The night sky from the Arctic
📸 credit-Giulio Cobianchi
location-Vareid beach,Lofoten Islands, Norway

"Blue moon"

A Blue Moon is a special type of full moon. Despite its name, the Moon usually does not appear blue. The term most commonly means:

The second full moon occurring within a single calendar month.

Why does it happen?

The Moon's cycle from one full moon to the next is about 29.5 days.

Because most months have 30 or 31 days, occasionally a month can fit two full moons:

First full moon: early in the month

Second full moon: near the end of the month

When this happens, the second one is called a Blue Moon.

Connection with 31 May 2026

May 2026 had:

A full moon on 1 May 2026 (the Flower Moon)

Another full moon on 31 May 2026

Since May contained two full moons, the full moon on 31 May 2026 was officially a Blue Moon.

An extra rarity: a Micromoon

The 31 May 2026 Blue Moon was also a Micromoon.

A micromoon occurs when the full moon happens near the Moon's apogee—the point where it is farthest from Earth. As a result, it appears slightly smaller and dimmer than an average full moon.

Does a Blue Moon look blue?

Usually no.

The name refers to the timing of the full moon, not its color. Only very unusual atmospheric conditions—such as volcanic ash or smoke particles in the atmosphere—can make the Moon actually appear bluish.

Why is the phrase “once in a blue moon” used?

Blue Moons are relatively uncommon, occurring about every 2–3 years. That's why English uses the expression:

“Once in a blue moon” = something that happens very rarely.

Timeline for May 2026

🌕 1 May 2026 → First full moon (Flower Moon)
🌕 31 May 2026 → Second full moon (Blue Moon) + Micromoon

Interestingly, the Blue Moon of 31 May 2026 was the only calendar Blue Moon of 2026, making it one of the notable sky events of the year.

Comet ZTF Rising Behind Yosemite Falls
📸 credit-Tara Mostofi
Location-Yosemite National Park, California, USA
Details: Canon EOS Ra Sigma 120-300mm , f/2.8, 27 Jan 2022 foreground 20:00, ZWO AM5 mount, APT, PHD2;
Planned with Photopills and Stellarium. Processed with PixInsight and Photoshop

Explanation - The Astronomer

Comet ZTF usually refers to C/2022 E3 (ZTF), a famous "green comet" that attracted worldwide attention in early 2023.

ZTF stands for the Zwicky Transient Facility, a sky-survey telescope system at Palomar Observatory. It discovered the comet on 2 March 2022.

Why was it called the “green comet”?

The comet developed a bright green glow around its nucleus. This color comes mainly from diatomic carbon (C₂) molecules that become excited by ultraviolet sunlight and emit green light. The green color is common in gas-rich comets, but it is usually difficult to see unless a comet comes relatively close to Earth.

Where did it come from?

Scientists believe it originated in the distant Oort Cloud, a huge shell of icy bodies surrounding the Solar System. It is classified as a long-period comet, with an estimated orbital period of about 50,000 years.

Why was it special?

It passed closest to Earth on 1 February 2023 at about 0.28 AU (around 42 million km).

It became bright enough to be seen with the naked eye under very dark skies.

Many reports noted that the last time it visited the inner Solar System was around the time when Neanderthals were still living on Earth.

Orbit

The comet follows a highly elongated orbit:

e~0.999988

An eccentricity this close to 1 means the orbit is extremely stretched, carrying the comet from the far outer Solar System toward the Sun and then back outward.

Structure of the comet

A comet like C/2022 E3 (ZTF) has:

1. Nucleus – icy rocky core.

2. Coma – glowing cloud of gas around the nucleus (the green part).

3. Dust tail – reflects sunlight.

4. Ion tail – made of charged particles pushed away by the solar wind.

Will it return soon?

Probably not. Estimates suggest another return would take tens of thousands of years, and some orbital calculations even indicate it may eventually be ejected from the Solar System after future gravitational interactions.

In short, Comet C/2022 E3 (ZTF) was a rare long-period green comet from the Oort Cloud that made a spectacular pass near Earth in 2023, giving astronomers and skywatchers one of the most memorable comet sightings of recent years.

Comet C/2025 A6 (Lemmon)
📸 credit- redit: Ralf Rohner
Location -Sierra Nevada, USA

on the Foothills of Mount Everest
Camera 📸 credit- Luo Hongyang
Location -Xizang Autonomous Region, China