Astronomy Picture of the Day

Explanation: September's equinox arrives today at 0905 UT. As the Sun crosses the celestial equator heading south, spring begins in the southern hemisphere and autumn in the north. And though the seasonal connection is still puzzling, both spring and autumn bring an increase in geomagnetic storms. So as northern nights grow longer, the equinox also heralds the arrival of a good season for viewing aurora. Recorded earlier this month, these curtains of September's shimmering green light sprawl across a gorgeous night skyscape. In the foreground lies Hidden Lake Territorial Park near Yellowknife, Northwest Territories, Canada. Calm water reflects the aurora, with bright star trails peering through the mesmerizing sky glow. Of course, shining at altitudes of 100 kilometres or so, planet Earth's aurorae are visible from space.