Harsh summer sun, sticky heat and exhausted daily life soaked in sweat may make it seem as if the sun is very close to Earth. Yet when summer is at its peak in the Northern Hemisphere, our planet is actually at the farthest point in its orbit from the sun.
In astronomy, this distant cosmic moment is called aphelion. Every year in early July, many people suffer through heat for days. According to NASA, during aphelion in July the distance between the sun and Earth increases to about 94.5 million miles, or 152 million kilometers. By contrast, in early January, during the cold winter month in the north, Earth is closest to the sun, a point known as perihelion. At that time the distance falls to 91.4 million miles, or 147 million kilometers. The difference is about 3.1 million miles, or 5 million kilometers.
By ordinary logic, it may seem that just as being closer to a lamp or candle feels hotter, moving farther from the sun should make Earth cooler. But nature’s cosmic system follows different rules. The sun-Earth distance plays only a minor role in seasonal change; the real mystery lies in Earth’s own axis.
Earth does not float through space perfectly upright. It is tilted, orbiting the sun with its rotational axis inclined at about 23.5 degrees, or 23.4 degrees, to the plane of its orbit. This permanent tilt creates the changes in temperature and weather across the planet.
When the Northern Hemisphere tilts toward the sun during Earth’s journey through space, summer arrives there. Because of the tilt, strong sunlight strikes the surface more directly. Days also become longer. With long periods of direct sunlight, large amounts of heat build up in the surface and atmosphere. So even when the sun is farther away, the intensity of its rays heats Earth.
The opposite happens when a hemisphere tilts away from the sun. Sunlight then falls at a slant. Slanting light spreads over a wider area, loses intensity and comes with shorter days, creating winter conditions. Because of Earth’s elliptical orbit, changes in distance do cause some variation in solar radiation. The 3.4 percent difference between perihelion and aphelion produces only about a 6.8 percent difference in the amount of solar energy reaching Earth.
If distance were the main factor, the whole Earth would experience extreme summer in July at the same time. In reality, that does not happen. When the Northern Hemisphere is facing intense heat, countries in the Southern Hemisphere such as Australia and South Africa are experiencing calm, cool winter.
বাংলায় মূল প্রতিবেদন পড়ুন · Read the original Bengali report