This spectacle will happen! Tonight! It will be visible from most of our country!

On the evening of March 24, 2023, a wonderful fairy tale of stars and moon will be staged in the Chinese sky. At 18:27 Beijing time, Venus will be in conjunction with the moon . Venus as seen from the center of the earth will be 0.11 degrees north of the moon. The moon will occult Venus in China, India, Africa and other places.

The moon occulted Venus on December 8, 2015. Image credit: NASA/Joel Kowsky

This is a great opportunity for China. It is the third day of the second lunar month. The moon is a crescent moon, 9.2% illuminated, and the brightness of Venus is around -4. In central and southern China, you can see bright Venus being occulted by the dark edge of the moon and then reappearing by the bright edge. In northern China, you can see the moon passing not far south of bright Venus .

In astronomy, when the apparent ecliptic longitude or apparent right ascension of two celestial bodies is equal, it is called conjunction. Conjunction usually means that the angular distance between the two celestial bodies is the smallest, and the two are closest. At the beginning of the month, Venus and Jupiter just passed the conjunction, and now the moon occults Venus again. It should be said that Venus is really busy in this March!

Image source: in-the-sky

Image source: interesting-sky

01

Moon occults Venus

How do you cover it up?

The so-called lunar occultation of Venus is the phenomenon that the moon, in its orbit, blocks Venus in the line of sight. In its orbit, the moon not only occults Venus, but may also block other celestial bodies, resulting in the phenomenon of "lunar occultation of stars"*.

The process of the moon occulting Saturn, photographed in Austria on November 3, 2001. Image source: Wikipedia

This type of astronomical phenomenon is not rare. Because all the planets and the moon in the solar system are orbiting near the ecliptic plane (that is, the plane of the Earth's orbit around the Sun), and the apparent diameter of the moon (relative to the visual angle of Earth observers) reaches 30 arc minutes, it orbits the celestial sphere once a month, while other planets move relatively slowly. In addition, the diameter of the Earth is relatively large relative to the distance between the Earth and the Moon, and the parallax of the position of the Moon seen by people in different regions is also large. Therefore , it is a common phenomenon to see the Moon blocking the planets or stars behind it in a certain area of ​​the Earth .

02

Common but hard to appreciate

Why is this celestial phenomenon so disrespectful?

The last time the moon occulted Venus in my country was on May 27, 2022. Although it was daytime, Venus was very bright and at a large angular distance from the sun, so it could still be observed with a telescope. The next time the moon occulted Venus in my country will be on the evening of September 26, 2026, but it will only be visible in southwestern my country, such as Yunnan and Guangxi.

For regions around the world, the frequency of lunar occultation of Venus is higher (occurring almost once or twice every year), the last time being on May 27, 2022; the next time will be on November 9, 2023, and will be visible in western Asia, Europe, North Africa and other places.

Image source: interesting-sky

During the occultation of Venus by the Moon, although people on Earth see the Moon gradually approaching Venus and covering it, the actual distance between the two is very far, and this occlusion will not have any effect on the orbit of the celestial body. Unlike when the Moon occults a star, the occulted star usually "fleet" and then "quickly reappears", when occulting a celestial body with a relatively large apparent diameter in the solar system, such as Venus, Venus will be gradually covered by the edge of the Moon, and it may take tens of seconds from the beginning to the complete occultation .

The lunar occultation of Venus is similar to a solar eclipse. Each time, only people in certain parts of the Earth can see it. This area is called the occultation belt. This time, the belt width is equivalent to the diameter of the moon, with the widest point from north to south being about 3,472 kilometers, and the east-west length being more than 10,000 kilometers. The lunar occultation of Venus begins at different times in different regions, earlier in the west than in the east. As the moon moves around the Earth from west to east, African observers in the westernmost part of the occultation belt will first see the lunar occultation of Venus at around 16:00 Beijing time, while observers in southern Xinjiang, my country, will see it at around 19:00, and Nanjing, located further east, will see it at around 20:00.

Since both Venus and the moon are relatively bright, they can be seen even in the city. As long as the weather is clear, choose a place in the west with no obstructions to enjoy this astronomical phenomenon.

03

In addition to watching the fun

There are many tricks in the occultation wonders!

In fact, eclipses between celestial bodies are very common and have certain scientific value. In addition to lunar occultations, planets or asteroids in the solar system can also occult distant stars, and even stars can occult each other, such as Algol. This type of astronomical phenomenon is collectively called occultation in astronomy.

The lunar occultation of Jupiter on July 15, 2012, photographed in Italy. Image source: Wikipedia

Occultations are not only ornamental, but also of great scientific value. For example, by observing the occultation of a star by a solar system object, we can determine whether the occulted star is a binary star, measure the diameter of the star, and possibly discover the structure around the solar system object , such as rings and satellites. The rings of Uranus were discovered accidentally through observing occultations.

On March 10, 1977, Uranus passed in front of a star, and an occultation of the star occurred. When observing this occultation, astronomers noticed that the star flickered several times when it entered and left behind Uranus. This phenomenon of the star appearing and disappearing meant that it was blocked by something, so astronomers speculated that Neptune had a ring structure, and eventually confirmed that it had at least four rings .

When the spectacle of the moon occulting Venus occurs, in addition to viewing it directly with the naked eye, you can also use professional telescopes or photographic equipment to observe it. Call your friends and get ready to watch this rare starry sky feast together!

Produced by: Science Popularization China

Author: Wang Kechao (Popular Science Supervisor of Purple Mountain Observatory, Chinese Academy of Sciences)

Producer: China Science Expo

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