August 25
The Lonely Star of the South
Sun Position
The Sun is in Virgo at around +9° declination. Autumn is six weeks off in the Northern Hemisphere; the Southern Hemisphere is less than a month from the spring equinox.
Sky Highlight
Voyager 1 crossed the heliopause (the boundary between the solar wind and interstellar space) on or around August 25, 2012, a date confirmed by NASA in 2013 from analysis of plasma wave data. While not an annually recurring sky event, this date marks a useful fixed point in the calendar for thinking about the outer boundary of our solar neighborhood.
Deep Sky Object
NGC 7293, the Helix Nebula in Aquarius, is the largest apparent-size planetary nebula in the sky (spanning half a degree, comparable to the full Moon) and lies only about 650 light-years away. It is best seen from the Southern Hemisphere and low northern latitudes, where Aquarius rises high; from northern latitudes above 50°, it barely clears the horizon.
Featured Star
Fomalhaut, the A3Va main-sequence star in Piscis Austrinus at 25.1 light-years, is famous for the sharply resolved debris disk imaged by the Hubble Space Telescope in 2008, within which an apparent planet (dubbed Fomalhaut b) was detected. It rises in the southeast in late August evenings and is the brightest star in that part of the sky, lending Piscis Austrinus its only naked-eye marker.
Around This Date
- August 25, 1981Voyager 2 made its closest approach to Saturn, delivering the highest-resolution images of the rings and moons yet obtained and discovering complex structure in the ring system that defied existing models.
- August 25, 1989Voyager 2 made its closest approach to Neptune (the final planetary flyby of the Grand Tour) passing within 5,000 kilometers of the cloud tops and discovering the giant storm system later named the Great Dark Spot.
Fomalhaut is one of the nearest stars with a confirmed debris disk, which means it is already doing, 25 light-years away, what our own solar system did 4.5 billion years ago.