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> No Spring Picnic on Neptune

SEPT. 2003

No Spring Picnic on Neptune


Neptune's spring: Three Hubble photos taken over a six-year period

Imagine a spring where flowers don’t bloom, birds don’t chirp, and children don’t run outside to play. Welcome to springtime on Neptune.

Take your parka. Neptune’s springtime weather brings blustery storms, temperatures of minus 400 degrees Fahrenheit at the cloud tops, and fierce winds that sometimes gust to 900 miles per hour. What is remarkable is that Neptune — the farthest and coldest of the major planets — exhibits any evidence of seasonal change. After all, the Sun is 900 times dimmer than it is on Earth.

A warming trend is on the way

Neptune's distance from the Sun


So, how can astronomers tell that springtime has arrived at all? Researchers at the University of Wisconsin- Madison and NASA's Jet Propulsion Laboratory in Pasadena, Calif., used NASA’s Hubble Space Telescope to study the planet over time, making three sets of observations in six years. The images reveal that the bands of clouds encircling the planet’s southern hemisphere increase in brightness over time. Astronomers believe the cloud bands are getting brighter because the Sun is warming the atmosphere in the south more than in the north. The amount of sunlight each hemisphere receives at a given time plays a major role in determining Neptune’s seasons.

A slant on the seasons

Tilting toward spring


Seasons on Neptune occur for the same reason as on Earth. The seasonal changes on both planets occur because their axes tilt slightly. Earth is inclined 23.5 degrees. Neptune is tipped at an even greater angle: 29 degrees. As both planets circle the Sun, one hemisphere is always tipped toward the Sun; the other is tilted away from the Sun. When the southern hemisphere tips toward the Sun, it receives more sunlight than the northern hemisphere. That means it’s summer in the south and winter in the north. The opposite is true when the northern hemisphere is tilted toward the Sun. The north receives more sunlight, which means it’s summertime.

Unlike Earth, Neptune’s seasons last for years, not months. A single season on the planet, which takes almost 165 years to orbit the Sun, can last more than 40 years.


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