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NASA Astronomy Picture of the Day:

very 15 years or so, Saturn's rings are tilted edge-on to our line of sight. As the bright, beautiful ring system grows narrower and fainter it becomes increasingly difficult to see for denizens of planet Earth. But it does provide the opportunity to watch transits of Saturn's moons and their dark shadows across the ringed gas giant's still bright disk. Of course Saturn's largest moon Titan is the easiest to spot in transit. In this telescopic snapshot from July 18, Titan itself is at the upper left, casting a round dark shadow on Saturn's banded cloudtops above the narrow rings. In fact Titan's transit season is in full swing now with shadow transits every 16 days corresponding to the moon's orbital period. Its final shadow transit will be on October 6, though Titan's pale disk will continue to cross in front of Saturn as seen from telescopes on planet Earth every 16 days through January 25, 2026.

Photo by Every 15 years or so

Composite image of Ganymede, the largest and most massive moon of Jupiter, taken by the Juno spacecraft on this day in 2021.

NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill, CC BY 2.0, via Wikimedia Commons. View source.

“The past only comes back when the present runs so smoothly that it is like the sliding surface of a deep river,” Virginia Woolf wrote some years before she filled her coat-pockets with stones, waded into the River Ouse near her house, and, unwilling to endure what she had barely survived in the past, slid beneath the smooth surface of life."  More at The Marginalian ➜

NASA Astronomy Picture of the Day:

Have you ever seen a fireball? In astronomy, a fireball is a very bright meteor -- one at least as bright as Venus and possibly brighter than even a full Moon. Fireballs are rare -- if you see one you are likely to remember it for your whole life. Physically, a fireball is a small rock that originated from an asteroid or comet that typically leaves a fading smoke trail of gas and dust as it shoots through the Earth's atmosphere. It is unlikely that any single large ground strike occurred -- much of the rock likely vaporized as it broke up into many small pieces. The featured picture was captured last week from a deadwood beach in Cape San Blas, Florida, USA. Piece it Back Together: Astronomy Puzzle of the Day

Photo by Jason Rice

The stairs of the observation tower Alpenblick in Ulrichsberg, Upper Austria, seen from the basement of the tower.

Z thomas, CC BY-SA 4.0, via Wikimedia Commons. View source.

NASA Astronomy Picture of the Day:

Can some supernovas explode twice? Yes, when the first explosion acts like a detonator for the second. This is a leading hypothesis for the cause of supernova remnant (SNR) 0509-67.5. In this two-star system, gravity causes the larger and fluffier star to give up mass to a smaller and denser white dwarf companion. Eventually the white dwarf's near-surface temperature goes so high that it explodes, creating a shock wave that goes both out and in -- and so triggers a full Type Ia supernova near the center. Recent images of the SNR 0509-67.5 system, like the featured image from the Very Large Telescope in Chile, show two shells with radii and compositions consistent with the double detonation hypothesis. This system, SNR 0509-67.5 is also famous for two standing mysteries: why its bright supernova wasn't noted 400 years ago, and why no visible companion star remains.

An Airstream Safari trailer in Joshua Tree National Park at dusk. Airstream founder Wally Byam died on this day in 1962.

Dllu, CC BY-SA 4.0, via Wikimedia Commons. View source.

Mary Ann Evans (1819 - 1880) Also known by her pen name George Eliot, was one of the leading English writers of the Victorian era .