Chinanews Client Beijing, March 25th (Reporter Shangguan Yun) On the 24th, for the first time this year, "Mercury West Large Distance" was staged in the sky, which attracted the attention of many astronomy enthusiasts, and some people said that this was an "astronomical wonder".

However, Dr. Jie Zheng, an assistant researcher at the Xinglong Observation Base of the National Observatory, said in an interview with the reporter of Chinanews.com that there are usually 6-7 Mercury distances, including the East and West distances. Simulation of the revolutions of ”can calculate the time of each large distance.”

Great opportunity to watch Mercury

In the solar system, Mercury is the planet closest to the sun. It orbits the sun once every 87.968 earth days, and it also rotates 3 times per 2.01 revolutions.

Mercury topographic map part. (Data map) Image source: NASA official website

At about 5 am on March 24, Mercury rises from the horizon. This is also Mercury's first "western distance" this year, which provides a very good viewing opportunity for people.

Within one week before and after this large west event, Mercury's magnitude will reach +0.2, and it will quickly drown in the sky before sunrise. From 5:30 am to 6:00 am, people can look through their eyes or ordinary binoculars. Telescope admires Mercury.

However, Zheng Jie said that although Mercury's western long-distance angular distance reached 27.8 °, which is relatively large, Mercury is currently located to the south, and Mercury's ground level angle is still very small at sunrise.

"The angle of ground level is only about 10 ° for Beijing, which is relatively low. Therefore, it is easily obscured by ground buildings and plants," he said.

What is east distance and west distance?

Some people are curious about the meaning of "large distance" as the astronomical phenomenon of "mercury large distance". In practice, this means that the angular distance between Mercury and the Sun is at its maximum.

"When we measure objects on the ground, it is natural to use the unit of length to express the distance or size between them, but it is different when looking at the sky." Zheng Jie said, the distance between heavenly bodies, we When observing, they also pay attention to their open angle.

Schematic diagram of Venus east long distance and Mercury west long distance in March 2020. Yuan Fengfang, Science Popularization Supervisor of Xinglong Observation Base of National Astronomical Observatory

As for the calculation of Mercury's "large distance", Zheng Jie explained that planets such as Mercury and Venus whose orbits are located within the Earth's orbit, due to the difference between the orbital period and the Earth's orbital period, it seems that their positions relative to the sun on the earth are constantly changing of.

"This change has a period. But because the planet's orbit is elliptical, the period is relatively stable, but not fixed, and there are usually 6-7 large distances each year, including east and west distances. Zheng Jie said, "We can simulate the revolution of Earth and Mercury, and then we can calculate the time of each large distance."

Mercury is called the East Large Distance to the east of the Sun and west to the West. At large distances, Mercury appears above the western horizon at dusk; at large distances, Mercury appears at low altitudes in the east at dawn.

Why is it difficult to observe?

Previously, there was a saying that from the perspective of Earth, Mercury is a relatively difficult planet to observe. Zheng Jie said that this was caused by the too close distance between Mercury and the Sun.

"Take this large western distance as an example, Mercury appeared on the horizon before sunrise. Or, early observation, the sky is dark, but Mercury's horizon level is very low; or, late observation, Mercury's horizon level is high, but the sun is also Rising, the sky's background is very bright. Therefore, Mercury is more difficult to observe. "Zheng Jie said.

"In stark contrast is Venus. In addition to being bright, Venus has a large angle at large distances. Venus reached the eastern distance on March 25. After dusk and sunset in recent days, we can all see the West Bright Venus in the sky, "Zheng Jie said.

Except for launching probes to Mercury for direct observation, the observation of Mercury on Earth is not much different from other astronomical observations. It is nothing more than visual observation, science telescopes, professional astronomical telescopes, and so on.

"Mercury seen with a telescope will be brighter than it can be seen with eyes, but ordinary telescopes may not be able to see the details of Mercury. The aperture and magnification of the objective lens determine how much detail is seen." Zheng Jie explained.

Mercury's "History of Observing History"

On May 10, 2016, NASA announced the first complete Mercury topographic image drawn on the official website using the latest PDS technology. This complete topographic map shows the surprising geographic details of Mercury's surface, which also helps scientists to understand the history of Mercury's geochemical performance more comprehensively. (Data map) Image source: NASA official website

In fact, ancient people have observed and understood Mercury. In ancient China, a day of the week was divided into 12 days. It is probably divided into 12 parts from the east to the west along the equator of the sky, which is represented by the twelve names in the square of the land.

"If the day of the week (360 °) is divided into 12 days, 30 ° per hour, Mercury is called Chenxing because Mercury always seems to oscillate on both sides of the sun, and the distance from the sun is no more than 30 °." Zheng Jie said.

In modern times, after precise observations of Mercury, it was found that Mercury's orbit has a slow precession, that is, the meridian point of Mercury's orbit is constantly changing. This cannot be explained by classical Newtonian mechanics.

"People even thought that there was still an undiscovered planet within Mercury, and it was thought that its gravity caused Mercury's abnormal orbit." Zheng Jie said, and later, Einstein's general theory of relativity solved this problem satisfactorily, "Mercury's precession has also become one of the three major evidences of general relativity." (End)