Earth, from a distance

A view of Earth from the EPIC camera aboard the DSCOVR satellite.

This month NASA released a time-lapse video (link) of the Earth, taken over the past year. The 3,000-plus images that make up this video were captured by the EPIC camera, located on the DSCOVR satellite. To take these images the camera has to be positioned at just the right location.

This satellite is orbiting the Sun, not the Earth, and it is a million miles closer to the Sun than the Earth. That should create a problem: Kepler’s third law of planetary motion tells us that an object closer to the Sun will orbit faster than the Earth, causing the distance between them to increase over time. But to collect the images needed for this video, the relative positions of the satellite and the Earth needed to stay the same for the entire year.

As an object moves closer to the Sun, the gravitational pull increases. The object therefore also needs to orbit faster to compensate for this increase in pull. If the object’s orbital speed did not increase, it would fall into the Sun.

But if the satellite is positioned at just the right distance between the Earth and the Sun, the opposing pull of the Earth will reduce the effective pull of the Sun to be the same as the Sun’s pull on Earth. This will allow the satellite to orbit the Sun at the same speed as the Earth while also maintaining a closer orbit.

This location is the L1 Lagrange Point, a point located on a line between the Earth and the Sun at a distance of about one million miles from Earth. At this point the satellite’s orbital motion matches that of the Earth, so the two stay together throughout the Earth’s orbit around the Sun. The EPIC camera is located at this position.

There are five Lagrangian points, L1 — L5, in the Earth-Sun system. They were named for an Italian-French mathematician who wrote about them in a 1772 paper concerning what he called the “three-body problem.” L1, which is the location of the camera for this video, is about four times farther from Earth than the Moon and about one one-hundredth of the distance to the Sun.

The EPIC camera takes a new set of images every two hours. Each time, it takes a series of ten images, using a different filter for each image. The filters range from ultraviolet to near infrared.

These images are used to produce a variety of science products. The images from the red, green, and blue filters are used to produce a color photographic-quality image of the Earth. This color image is our best estimate of what a human sitting at the location of EPIC would see.

The images also capture the ever-changing motion of clouds and weather systems and the fixed features of Earth such as deserts, forests, and blues of different seas. In addition, EPIC allows scientists to monitor ozone and aerosol levels in Earth’s atmosphere, cloud height, vegetation properties, and the ultraviolet reflectivity of Earth. The images can also be used to produce global maps of dust and volcanic ash.

The DSCOVR satellite is a partnership between NASA, the National Oceanic and Atmospheric Administration, and the U.S. Air Force. The primary objective of the mission is to monitor solar activity in real time, including the solar wind. This information is critical to the accuracy and lead-time of the space weather alerts and forecasts provided by NOAA.

During the past year EPIC captured several events. In March it captured the shadow of the Moon passing across the surface of the Earth. This shadow was caused by a total solar eclipse. The people on earth who were in the path of the shadow may have been able to see a total solar eclipse if the skies were clear.

EPIC also captured some lunar transits. Because DSCOVR is always between the Sun and the Earth, and because the Moon orbits the Earth, you would think that the camera would always be able to see the Moon. But because EPIC has a very small field of view and the Moon orbits at a great distance from the Earth, the Moon is rarely seen in any of the images. The five-degree tilt of its orbit means that even when the Earth, Moon, and EPIC are in line, the Moon is above or below the field of view. EPIC therefore captured only two lunar transits — one on July 16, 2015, and the other on July 4 of this year.

With DSCOVR’s mission life set for a total of five years, we should expect many more sunlit global images and maybe even a few more movies.

Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at marty.scott@wallawalla.edu

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