March 29, 2023

Tomorrow the ‘green comet’ will make its closest approach to Earth

Jose Miguel Vinas Jose Miguel Vinas Weather Spain 6 minutes
green comet
Comet c/2022 E3 (ZTF) photographed by the Dark Sky Observatory®, Alqueva, Portugal on January 19, 2023. Author: Miguel Claro. Source:

Tomorrow, Wednesday (February 1, 2023) the ‘green comet’ will make its closest approach to Earth: about 42 million kilometers. It’s been a few days now visible to the naked eye (as a magnitude +5 astronomical object) from the Northern Hemisphere, and in the coming days it will also be visible to the naked eye from the Southern Hemisphere. Under optimal conditions of visibility in the night sky, the human eye is capable of observing stellar objects up to +6.

It is already visible to the naked eye as a magnitude +5 astronomical object, with the human eye capable of observing stellar objects up to magnitude +6.

The comet came to be called “green,” as that is the striking color of its coma (the gaseous cloud surrounding its nucleus) in astrophotographs. Item listed as c/2022 E3 (ZTF). The reference to 2022 is because were discovered last year (March 2) by a pair of American astronomers using a wide-field camera at the Palomar Observatory, near San Diego, California, USA, as part of an automated sky-watching program called Zwicky transit facilityhence the acronym ZTF, which also includes the comet’s technical name.

The comet’s nucleus is just under 2 kilometers in diameterbut when it got closer to the Sun (it reached its perihelion on January 12), it began to become more visible from Earth, thanks to the detachment of many tails that reach a length of several million kilometers.

The comet has a very eccentric orbit and the last time it approached Earth was 50,000 years ago, during the last ice age, when Europe was inhabited by Neanderthals (now in decline) and homo sapiens. It is fascinating to know that on some cold, dark nights back then, our ancestors saw this same green comet in the sky.

Comets throughout history

The truth is that hundreds of comets have appeared in the sky throughout human history. For a long time they were interpreted as harbingers of calamity. In ancient Greece, Aristotle in his “Meteorological” treatise (4th century BC) mistakenly attributed an atmospheric nature to them. (contrary to the ideas of other Greek philosophers such as Democritus), and this error continued for many centuries, until finally, thanks to the development of Astronomy, we understood that they were objects of extraterrestrial origin traversing the world.

Great Comet of 1577
The Great Comet of 1577, seen in Prague on November 12 of that year. Engraving by Jiri Daschitzky. Source: Wikipedia

According to Aristotle, comets formed in the sublunar region.which was one of the regions into which the sky was divided during the classical era, with the regions associated with each of the four basic elements of nature (air, water, earth, and fire) below, and the spheres of the planets and the treasury of the sky full of stars above.

It was not until the sixteenth century that after the passage of the Great Comet of 1577, the Danish astronomer Thycho Brahe (1546-1601) correctly concluded that it and other comets were astronomical objectsperiodically approaching Earth, as is the case today with Comet c/2022 E3 (ZTF).

How to observe the “green comet” (ZTF)?

As we mentioned, although the comet has been visible for a few days in the northern hemisphere, light pollution in our cities and the presence of the moon during the morning make it difficult to observe with the naked eye the “little cloud” (a small patch of diffused light) that we can appreciate, so it’s good to use binoculars (mounted on a tripod) or a small telescope, so we can see its long tail.

To observe it successfully, the first thing you need to do is quite far from cities or population centers, seeking the darkest sky. It is also necessary avoid lunar hours, for which we have to get up early and make the observation during the two to three hours before dawn. Of course, the sky must be cloudless, which this week is guaranteed in almost all of Portugal. We should direct our gaze (binoculars or telescope) to the north and specifically to the Little Bear. We will see the comet not far from its brightest star, Polaris.though in the coming days it will move away towards the neighboring constellation of Auriga (The Charioteer), and on February 5th it will be very close to the bright star Capela.

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