Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
The view is different for each of us, but this perspective unites us all.
From anywhere on Earth, the entire sky rotates a full 360° every 24 hours.
From Earth, star trails always appear to rotate around the celestial poles. From the Northern Hemisphere, the bright star Polaris can be easily identified as the small streak located just 1 degree from the true celestial north pole. In the southern hemisphere, there is no such bright pole star. (Alan Dyer /VW PICS/Universal Images Group via Getty Images)
A skyward-pointed camera with the shutter left open will capture curved streaks: star trails.
Above the central array of the Atacama Large Millimetre/Submillimetre Array (ALMA), the southern celestial pole can be pinpointed as the point about which the other stars all appear to rotate. The length of the streaks in the sky can be used to infer the duration of this long-exposure photograph, as a 360 degree arc would correspond to a full 24 hours of rotation. (ESO/B. TAFRESHI (TWANIGHT.ORG))
From the International Space Station’s perspective, a far more spectacular view awaits.
The ISS orbits so rapidly that it completes a revolution around Earth every 90 minutes.
Astronaut Karen Nyberg looks out the cupola window aboard the International Space Station (ISS). The ISS has been operational for over 20 years, and even though its total cost has been 150 billion USD, that equates to just 0.2% of the US government’s budget per year: a cost that was split between many different countries and space agencies. (NASA / JOHNSON SPACE CENTER)
Since 2010, ISS astronauts have had access to a “cupola,” granting direct, wide-field views of Earth.
German astronaut Alexander Gerst took this spaceborne image of a spectacular Earthly aurora from aboard the International Space Station. Portions of the external solar panels can be seen at right, while the enormous vertical extent of the aurora, along with the varied colors produced, are highlighted from this towering perspective. (Alexander Gerst / ESA via Getty Images)
It also gives the astronauts an opportunity to experiment with novel photography techniques.
Star trails as seen from the International Space Station as it rotates on its axis and revolves around the Earth. Numerous features of our planet, including the various glowing layers of the atmosphere as well as city lights and lightning strikes can be seen. The stars appear yellow, white, and blue, but the green glow at the top right is coming from a reflection of a light on board the ISS itself. (DON PETTIT / NASA / ISS EXPEDITION 31)
As the Space Station streaks from right to left in this photo, the streaks on the Earth below follow its motion relative to the planet. However, the Space Station itself rotates on an axis that points towards the central point about which the stars rotate, indicating a difference between these two motions. (DON PETTIT / NASA / ISS EXPEDITION 31)
By stacking a few dozen 30-second exposures together, he produced spectacular star-trails.
From a certain perspective, perpendicular to the ISS’s axis of rotation, star trails appear to rain down on Earth, while they appear to orbit about a single point closer when you look along the axis of rotation. Down below, only a few bright yellow streaks persist, a sign of clear weather and sparse human populations below. (DON PETTIT / NASA / ISS EXPEDITION 31)
Down below, the Earth appears to move relative to the ISS, creating ground streaks.
A combination of factors make Earth look like some sort of cosmic disco ball in this photograph composite from Don Pettit aboard the International Space Station. The blue dots are from lightning strikes, the greenish covering is from atmospheric airglow, while the yellow streaks are due to the rapid motion of lights on Earth’s surface. The star trails light up the background. (DON PETTIT / NASA / ISS EXPEDITION 31)
Meanwhile, the stars streak not around the Earth’s axis, but around the ISS’s (arbitrary) axis of rotation.
The star trails from the ISS appear to streak very rapidly, owing to the relatively rapid rotation of the station itself. Because of its intense motion around the Earth, the station is configured to rotate a full 360 degrees every 90 minutes or so, keeping in time with the orbit of the station around Earth. The atmospheric airglow is clearly visible above the ground. (DON PETTIT / NASA / ISS EXPEDITION 31)
The green and red layers above Earth’s atmosphere are airglow, visible everywhere.
The bright yellow streaks below all correspond to artificial night lighting on Earth, including stationary city and highway lights, moving sources like vehicles, and even bright offshore light sources such as those arising from squid fisheries. Above, the atmospheric airglow can be seen, with star trails in the background showing the ISS’s axis of rotation. (DON PETTIT / NASA / ISS EXPEDITION 31)
The yellow streaks are caused by artificial lighting, while blue “dots” arise from lightning strikes.
The large number of blue dots seen in this image each correspond to a brief but powerful lightning strike occurring on Earth. The fact that this is a 10–15 minute composite of exposures showcases just how violent some of these thunderstorms occurring on Earth can be. (DON PETTIT / NASA / ISS EXPEDITION 31)
Occasionally, a brilliant auroral display joins the show as well.
The brilliant green hue that covers the left portion of Earth’s atmosphere in this image isn’t just the typical airglow that can be seen at right, but an auroral display that arises from ionized electrons falling back onto the atoms and molecules in the atmosphere. These auroral displays take on an entirely new perspective when viewed from outer space. (NASA/DON PETTIT/EXPEDITION 31)
We’re all part of this one Earth, as captured from this unique vantage point.
A number of the ISS modules can be seen from aboard the space station itself, while the station both orbits the Earth and rotates on an axis all its own. Below, city lights, lightning strikes, and the atmospheric airglow can be seen, while the background stars streak through the sky: a truly breathtaking sight. (DON PETTIT / NASA / ISS EXPEDITION 31)
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.
