Three arches spread across the night sky above the snow-covered Alps. It is the "Astronomy Picture of the Day" that NASA released on the 21st local time. Angel Fux, a French astrophotographer, said she took a helicopter last month, landed on a peak in the Alps near the Switzerland-Italy border, and captured the scene.
Fux originally expected two Milky Way arches formed by our galaxy in the night sky. Our galaxy is a massive disk, and the solar system lies on its edge about 26,000 light-years (one light-year is the distance light travels in one year, about 9.46 trillion km) from the center. Therefore, from Earth we can see both the inner and outer parts of our galaxy.
The center of our galaxy lies toward Sagittarius. There, stars are densely packed, so the Milky Way appears large and bright. In the Northern Hemisphere in March, Sagittarius is near the southeastern horizon just before sunrise. The arch at the far left of the photo is the very core of our galaxy. The outer part of our galaxy lies in Auriga. In March, you can find it in the northwest sky after sunset. Because you are seeing the edge of the galaxy, the Milky Way appears faint and narrow.
Fux left the Alps by helicopter the next morning. After 40 hours of processing and compositing the images taken that night, she produced this 360-degree panorama of triple arches. The photographer found another arch that seemed to connect the two Milky Ways. It is the zodiacal light.
People long ago thought Earth was the center of the universe and that the sun and all celestial bodies revolved around it. In fact, Earth orbits the sun, but the apparent path along which the sun seems to move over a year, as seen from Earth, is called the ecliptic.
The zodiacal light is not a phenomenon produced in Earth's atmosphere but the result of sunlight scattering off dust outside Earth. Its color is therefore different. When the sun rises or sets near the horizon, its light travels a long path through the atmosphere. Short-wavelength blue light is scattered away, and only long-wavelength red light reaches our eyes. By contrast, the zodiacal light appears as a faint white glow.
Depending on when it is observed, the zodiacal light appears as false dusk or false dawn. In the Northern Hemisphere, false dusk is seen around the spring equinox between late February and early April, about 90 minutes after sunset, as a faint cone of light above the western horizon. False dawn is seen around the autumnal equinox between late August and early October, about 90 minutes to two hours before sunrise, above the eastern horizon. It can be seen on moonless nights far from city lights.
Scientists are studying where the dust that creates the zodiacal light comes from. By using the brightness distribution of the zodiacal light, they can learn how particles are distributed between planets. They can also study where this dust originated. For a long time, space dust causing the zodiacal light was thought to come from asteroids and comets, but recent research has suggested that the space dust responsible for the zodiacal light may have come from dust storms on Mars.