On this day, 115 years ago, on April 20, 1910 (March 11, 1910, the lunar calendar), Halley's Comet reached perihelion. Scientists first photographed and spectrally observed Halley's Comet. On April 20, 1910, Halley's Comet reached perihelion. Halley's Comet is the only large comet that can be predicted. In 1705, British astronomer Halley used Newton's law of universal gravitation to calculate its return period and orbit. In recognition of his achievements, he named the comet Halley. It has become a "regular customer" with 32 returns since 240 BC. Halley's Comet has a period of approximately 76 years. It appeared twice in the 20th century. The conditions were good at the time of the return in 1910, so the image was quite spectacular: the comet tail was so bright that it was visible to the naked eye when it passed the perihelion on April 20, and when it passed the perigee a month later, the comet tail was as long as 125 degrees to 150 degrees. At that time, since it was only 25 million kilometers away from the Earth, some people feared that all life on earth completely covered by the comet's tail would die. In fact, the tail of the comet is very thin, and nothing abnormal has happened to the earth. However, the scene of Halley's Comet sweeping across the sky really made people horrified at that time. British astronomer Halley