Spring Has Sprung …


… here in the Northern Hemisphere. The equinox occurred last week. At an equinox Earth’s terminator, the dividing line between day and night, runs through the planet’s north and south poles as seen at the start of this time-lapse video which crams an entire year into twelve seconds. It was put together using Meteosat infrared images taken every day at the same local time from a geosynchronous orbit. The video actually starts at the September 2010 equinox. As the Earth revolves around the Sun, the terminator tilts as less daily sunlight falls on the northern hemisphere, reaching the solstice and northern hemisphere winter at the maximum tilt. As the year continues, the terminator tilts back again to the March 2011 equinox halfway through the video. Then the terminator swings past the poles, until the June 2011 solstice, the start of northern summer. The video ends as the September equinox returns.

Video Credit: NASA / Meteosat / Robert Simmon

Seasons on Titan


Titan TemperaturesThis animated sequence of maps shows varying surface temperatures on Saturn’s moon Titan at two-year intervals from 2004 to 2016. The measurements were made by the Cassini spacecraft. They show heat coming from Titan’s surface at a wavelength of 19 µm, a wavelength at which the moon’s otherwise opaque atmosphere is somewhat transparent. Temperatures have been averaged around the globe from east to west to emphasize the seasonal variation across latitudes. Regions for which there are no data show up as black.

Titan’s surface temperature changes slowly over the course of the Saturn system’s long seasons, which each last 7-1/2 years each. As on Earth, the amount of sunlight received at any location changes as the Sun appears to move north or south in Titan’s sky over the course of the 30-year-long Saturnian year. Cassini arrived at Saturn in 2004 when Titan’s southern hemisphere was in late summer and was the moon’s warmest region. By 2010, shortly after the 2009 equinox, temperatures were about the same across the northern and southern hemispheres, similar to the situation seen by Voyager 1 in 1980, one Titan year earlier.

Image Credit: NASA