How can the naked eye see Venus?

Besides the moon, the brightest star in the night sky must be Venus. At night, it shines in the western sky, and is called the Pleiades. Sometimes, at dawn, he appears in the eastern sky and is called the morning star. The reason is that Venus is a planet in the solar system on earth, so wherever the sun goes, it follows. Although Venus is the brightest star in the night sky, Venus itself does not shine. Its bright brilliance is reflected sunlight. Just as moonlight is the reason why the moon reflects sunlight, Venus is so bright because its mass and volume are relatively large, similar to that of the earth. In addition, it is also the closest planet to the earth, with the farthest distance of no more than 260 million kilometers, compared with only 38 million kilometers recently.

But whether it is the farthest or the nearest, we can't see its glory. We can't see it because it is on the other side of the sun when it is farthest from the earth, and it is blocked by the brightness of the sun, but when it is closest to the earth, it will basically be in a straight line with the sun of the earth. At this time, the sun's light illuminates the side of Venus facing away from the earth. Of course, we can't see it, but sometimes we can see its shadow passing over the surface of the sun. This is the transit of venus phenomenon. Venus' apparent magnitude can be as high as -4.9, and no star on the earth can have such high brightness, so the brightness of Venus can exceed that of all stars except the moon and itself, which means that all stars are not as bright as Venus itself all day. When the luminosity of visible light is low, it is often around -3, which is brighter than the second brightest Jupiter (-2.5).

But Venus never gets close to the earth at its brightest, because the brightness of Venus is mostly on the side facing away from the earth at this time, so it doesn't look very bright. It is brightest only when the vertex of the included angle between Venus and the earth is more than 80 degrees and less than 170 degrees, as shown in the following figure: at this time, the brightness of the surface of Venus can be reflected more to the earth, so it looks brighter. When we fly into space, we will see that there are more stars in the universe than cow hair. This seems to be a point light source because it is far away from us. On the surface of the earth, due to the influence of atmospheric disturbance, starlight always flashes. Looking at it, you will feel that the sky is full of stars. In fact, every star in the night sky has different brightness. Therefore, astronomers invented the concept of magnitude to quantitatively describe the brightness of stars (celestial bodies). Temperatures include Celsius, Fahrenheit, etc. Similarly, due to different reference divisions, there are many types of magnitudes. Only the concepts of visual magnitude and absolute magnitude are introduced here.

Simply put, the apparent magnitude is the apparent brightness of celestial bodies seen by normal adults, which is related to the actual brightness of celestial bodies and the distance between celestial bodies and observers. Absolute magnitude is the apparent magnitude observed when a celestial body is 32.6 light years away from the Earth (10 second). Absolute magnitude can only describe a self-luminous celestial body, such as the sun, and there is no absolute magnitude for a celestial body that reflects light from other celestial bodies, such as the moon. In clear weather, the apparent magnitude of the full moon is-13, while the apparent magnitude of the midday sun is -27. Unless otherwise specified, magnitude generally refers to apparent magnitude. According to calculation, under the condition of clear weather and good atmospheric transmittance, the darkest stars that an adult with normal vision can see are six. Now, with the aggravation of light pollution and air pollution, only stars under 5 stars can be seen, while cities can only see some brighter stars. The earth rotates from west to east, so the stars on the earth are divided into northern and southern hemispheres, ignoring the distance relationship. These stars are all distributed on an imaginary celestial sphere. Astronomers have drawn the distribution map of these stars on the celestial sphere through long-term observation and given each star a unique name. According to the statistics of astronomers, there are more than 6000 stars in the northern and southern hemispheres.