Some stars and constellations are always present in the night sky. For example, Polaris has served as a steady guiding light for thousands of years in the Northern Hemisphere.
However, if you look closely at the sky each night, you may notice that some stars move from east to west from dusk to dawn, or change their position from season to season. For example, Orion disappears from view for part of the year. So why do some constellations remain visible all year round, while others disappear and appear depending on the season?
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To understand this, it helps to note that when we look at the night sky, we only see part of the universe. “Night is when the Earth turns away from the sun,” Valerie Rapson, an astronomer at the State University of New York at Oneonta, told Live Science. “And whatever the star is in that direction, that’s the star we’re going to see.”
Additionally, which constellations are visible depends on where you are on Earth. Stargazers in the northern hemisphere see stars and constellations above the equator, while observers in the southern hemisphere see stars and constellations south of the equator.
As the tilted Earth revolves around the sun over the course of a year, different parts of the universe come into view while other parts recede. For example, in the Northern Hemisphere, the Orion Belt, which looks like a line of three bright flashing stars in the constellation Orion, is most clearly visible in winter. However, in the summer, as the Earth orbits on the opposite side of the sun, Orion is in a part of the universe that is blocked by sunlight and cannot be seen from the northern hemisphere.
visible all year round
There are some constellations that do not disappear depending on the season. These are called circumpolar constellations and are located near the celestial poles. Although these constellations are always visible, they rotate and change positions throughout the year, Rapson explained.
For example, the Northern Hemisphere has Polaris, the North Star, almost directly above the North Pole. The Big Dipper is a well-known series of stars known as the asterisms in the constellation Ursa Major and orbits the skies of the Northern Hemisphere all year round.
There are very few such circumpolar constellations in the Southern Hemisphere, where Brown is based. “Heaven’s South Pole” [is] “The sky is a little bit empty,” he said, but the Southern Cross, a constellation of four bright stars, can be seen in the southern sky all year round.
The International Astronomical Union recognizes a total of 88 constellations in the night sky, and astronomers have mapped the regular movements of these constellations using celestial coordinates. There are now interactive digital star maps like Stellarium that help stargazers navigate and find specific constellations based on the time of year.
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wobbling earth
However, these stars and constellations won’t look like this forever. There is a slight wobble in the Earth’s rotation. This means that over thousands of years, the positions and movements of constellations and the sky change slightly, at least from Earth’s perspective. For example, the timing of the zodiac signs in the sky, the constellations along the equator, is now out of alignment with what it was thousands of years ago due to the Earth’s wobble.
Wobble also affects the North Star. For example, Polaris was not always a star in this position. In fact, in 2500 BC, it was occupied by the star Tuban in the constellation Draco. The North Star of the future will also change. “In 5,000 years from now, we may have a completely different Polaris, or technically no Polaris at all, or at least not an exact match,” Rapson said.
Understanding this nuance and knowing exactly when constellations appear above the horizon is important for astronomy. Brown, who studies the evolution of galaxies, noticed one interesting constellation.
“Sagittarius is the center of our galaxy,” he said. It is mainly visible in winter in the Southern Hemisphere (summer in the Northern Hemisphere) and is also right next to the Milky Way’s supermassive black hole. “So those who study the Milky Way’s black hole are doing so at certain times of the year. [the black hole] The sun doesn’t block you,” Brown said.
Therefore, learning how stars change with the seasons is an important part of the discoveries related to this mysterious celestial object and many others.
“What may seem a little confusing at first can become clearer if you shift your perspective a little,” says Brown. By understanding the night sky and deciphering the Earth’s movement in relation to the stars, we can understand more about our place in the universe and its wonders.
Can you name all the animals, objects, and mythical figures hidden in the night sky? Find out with our constellation quiz!
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