By Steve Edberg

The last lunar eclipse until 2014 will grace the sky on Saturday, Dec. 10. Steve Edberg, an astronomer at NASA’s Jet Propulsion Laboratory, shares the best viewing times and tips. For more lunar eclipse resources and to join NASA/JPL’s “I’m There: Lunar Eclipse” event, visit http://1.usa.gov/uBfAI8.

Keith Burns submitted this winning photo of the December 2010 lunar eclipse as part of NASA/JPL’s “I’m There: Lunar Eclipse” Wallpaper contest. Planning to “be there” for the Dec. 10, 2011 eclipse? Post your images on the Total Lunar Eclipse Facebook event page for a chance to have your photo become an official NASA/JPL wallpaper. Copyright: Keith Burns

In the hours before dawn on Saturday morning, December 10, early risers in about half of the continental U.S. will have a chance to see at least some of a total lunar eclipse – the last one until 2014. The moon will be low in the western sky, and a clear, flat horizon without obscuring trees, buildings or mountains will make viewing easier. The setting of the moon and brightening of the sky as the sun rises will make observing the eclipse more challenging than usual, but more interesting too.

The moon’s passage has stages as it goes through the layers of Earth’s shadow. The outer ring of the shadow is called the penumbra. An observer on the moon would see a partial solar eclipse while the moon is in the penumbra. The core of Earth’s shadow is called the umbra. Observers on the moon would see a total eclipse of the sun when the moon is in the umbra. The time of moonset and the moon’s position in the earth’s shadow affects the view of the various stages of the event for observers across the US.

Washington, D.C. and the Eastern Time zone: The moon is setting just when it first enters the outer ring of Earth’s shadow. (This is called first penumbral contact, 06:33 EST, 05:33 CST, 04:33 MST, 03:33 PST, 01:33 AHST.) Effectively, no eclipse is visible. Sorry.

Chicago and the Central Time zone: Moonset is just before the moon enters the dark core of Earth’s shadow (called first umbral contact, 06:45 CST, 05:45 MST, 04:45 PST, 02:45 AHST). Observers in this region might see some darkening of a small section of the moon, just before the moon dips below the horizon.

Albuquerque and the Mountain Time zone: The moon sets (06:52 MST) with about 65 percent of its surface in the core of Earth’s shadow. Observers in this region will be able to see the moon’s entry and motion through Earth’s shadow until the moon disappears.

Los Angeles and the Pacific Time zone: With some variation from San Diego to Seattle, observers with an ocean horizon will be able to see the moon completely covered by the core of Earth’s shadow. (This is called totality, beginning at 06:06 PST, 04:06 AHST.) Southern observers will see a race between the end of totality and moonset. Observers in the Pacific Northwest will see the moon begin to emerge from the shadow core, ending totality (at 06:57 PST, 04:57 AHST). For them, the moon goes down (07:46 PST) in partial eclipse.

Honolulu and the Hawaiian Islands: Observers will see all phases involving the shadow core. Moonset occurs (07:05 AHST) after the last umbral contact, during the ending phase as the moon is exiting the outer ring of Earth’s shadow.

Anchorage, Alaska and northwestern Canada: The complete eclipse — from shadow outer ring entry, the moon’s passage through the shadow core, and its exit from the outer ring — will be visible before sunrise. The long nights at these latitudes make this possible.

The last total lunar eclipse visible in the US was about a year ago. Some of us will see a partial lunar eclipse next June, but after that we all wait until April 14-15, 2014 to see the whole spectacle of the moon passing through Earth’s shadow.

By Dr. David Diner

Tiny airborne particles, or aerosols, can affect the appearance of the moon during a total lunar eclipse, sometimes giving it a reddish hue. Copyright Ian Sharp

A lunar eclipse occurs when the Earth is positioned between the sun and the moon. Although the Moon passes through the Earth’s shadow, the lunar disk remains partially illuminated by sunlight that is refracted and scattered by the Earth’s atmosphere.

Refraction is the bending of light that occurs when the rays pass through media of different densities (our atmosphere is more dense near the surface and less dense higher up). Scattering of sunlight by molecules of air also deflects the light into different directions, and this occurs with much greater efficiency at shorter (bluer) wavelengths, which is why the daylight sky appears blue. As we view the sun near sunrise or sunset the light traverses a longer path through the atmosphere than at midday, and when the air is relatively clear, the absence of shorter wavelengths causes the solar disk to appear orange.

Tiny airborne particles, also known as aerosols, also scatter sunlight. The relative efficiency of the scattering at different wavelengths depends on the size and composition of the particles. Pollution and dust in the lower atmosphere tends to subdue the color of the rising or setting sun, whereas fine smoke particles or tiny aerosols lofted to high altitudes during a major volcanic eruption can deepen the color to an intense shade of red.

If you were standing on the Moon’s surface during a lunar eclipse, you would see the Sun setting and rising behind the Earth, and you’d observe the refracted and scattered solar rays as they pass through the atmosphere surrounding our planet. Viewed from the Earth, these rays “fill in” the Earth’s shadow cast upon the lunar surface, imparting the Moon’s disk with a faint orange or reddish glow. Just as we sometimes observe sunrises and sunsets with different shades of orange, pink or red due to the presence of different types of aerosols, the color of the eclipsed lunar disk is also affected by the types of particles that are present in the Earth’s atmosphere at the time the eclipse occurs.

By Sami Asmar

In addition to the awesome views they offer, lunar eclipses have always provided scientific clues about the moon’s shape, location and even surface composition. Although there will continue to be opportunities for observers to examine and reflect on fundamental concepts about the moon, such as its origin and interior structure, more modern tools are aiding these observations.

When it comes to understanding what a moon or a planet is made of remotely — short of touching it or placing seismometers on its surface or probes below the surface — classical physics comes to the rescue. By measuring the magnetic and gravitational forces that are generated on the inside and manifested on the outside of a planet or moon, we can learn volumes about the structure of its interior.

A spacecraft in the proximity of the moon can detect these forces. In the case of gravity, the mass of the moon will pull on the spacecraft due to gravitational attraction. If the spacecraft is transmitting a stable radio signal at the time, its frequency will shift by an amount exactly proportional to the forces pulling on the spacecraft.

This is how we weigh the moon and go further by measuring the detailed distribution of the densities of mountains and valleys as well as features below the moon’s surface. This collection of information is called the gravity field.

In the past, this has lead to the discovery mascons on the moon, or hidden, sub-surface concentrations of mass not obvious in images or topography. If not accounted for, mascons can complicate the navigation of future landed missions. A mission, human or robotic, attempting to land on the moon would need to have a detailed knowledge of the gravity field in order to navigate the landing process safely. If a spacecraft sensed gravitational pull higher than planned, it could jeopardize the mission.

The Gravity Recovery and Interior Laboratory (GRAIL) mission, scheduled to launch in September, is comprised of twin spacecraft flying in formation with radio links between them to measure the moon’s gravity field globally. This is because a single spacecraft with a link to Earth would be obstructed when the spacecraft goes behind the moon, leaving us with no measurement for nearly half of the moon, since the moon’s far side never faces the Earth. The GRAIL technique may also reveal if the Moon has a core with a fluid layer.

So as you go out to watch the lunar eclipse on the night of Dec. 20, think about how much we’ve learned about the moon so far and what more we can learn through missions like GRAIL. Even at a close distance from Earth, the moon remains a mystery waiting be uncovered.