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A Brief History of the Blood Moon
Millions of people around the world will witness a rare celestial treat today as the moon—the shimmering celestial spotlight that normally illuminates the night sky—is turning an eerie shade of red. But don’t worry, this is all part of a natural phenomenon called a “blood moon.”
The term “blood moon” is used to describe the celestial orb’s appearance during a lunar eclipse, which occurs each time the Earth passes between the sun and the moon. During these passes, our planet’s shadow falls over the moon, blocking out the sunlight it normally reflects. Unlike a solar eclipse—when the moon passes between the Earth and sun, blotting out the star’s light—the moon does not turn dark, but instead appears as a deep red color.
This is due to atmospheric effects.
During a lunar eclipse, the Earth blocks the majority of the sun’s light, but not all of it. Some light still touches the moon’s face. As the light from the sun’s rays travel through Earth’s atmosphere, most of the colors in the visible spectrum are filtered out due to scattering. Only the red and orange wavelengths are able to reach the lunar surface, giving it that reddish hue and earning it the title of “blood moon.”
Today’s eclipse marks the second appearance of a blood moon so far this year, and this one’s a bit unique as it also happens to be the longest lunar eclipse of the century.
Spanning Friday night and Saturday morning, the moon will be totally eclipsed by the Earth for one hour and 43 minutes. During this brief period, the moon will glow an eerie shade of red, so be sure to go outside and look up.
Thanks to science, we know that this seemingly strange phenomenon is actually quite common—and benign. Today, such celestial events are a cause for celebration with viewing parties, road trips and astronomy talks. But that wasn’t always the case.
Moons can also form at the same time as their parent planet. Under such an explanation, gravity would have caused material in the early solar system to draw together at the same time as gravity bound particles together to form Earth. Such a moon would have a very similar composition to the planet, and would explain the moon's present location. However, although Earth and the moon share much of the same material, the moon is much less dense than our planet, which would likely not be the case if both started with the same heavy elements at their core.
In 2012, researcher Robin Canup, of the Southwest Research Institute in Texas, proposed that Earth and the moon formed at the same time when two massive objects five times the size of Mars crashed into each other.
"After colliding, the two similar-sized bodies then re-collided, forming an early Earth surrounded by a disk of material that combined to form the moon," NASA said. "The re-collision and subsequent merger left the two bodies with the similar chemical compositions seen today.
It wasn't until the Ottoman Empire that the crescent moon and star became affiliated with the Muslim world. When the Turks conquered Constantinople (Istanbul) in 1453 CE, they adopted the city's existing flag and symbol. Legend holds that the founder of the Ottoman Empire, Osman, had a dream in which the crescent moon stretched from one end of the earth to the other. Taking this as a good omen, he chose to keep the crescent and make it the symbol of his dynasty. There is speculation that the five points on the star represent the five pillars of Islam, but this is pure conjecture. The five points were not standard on the Ottoman flags, and are still not standard on flags used in the Muslim world today.
For hundreds of years, the Ottoman Empire ruled over the Muslim world. After centuries of battle with Christian Europe, it is understandable how the symbols of this empire became linked in people's minds with the faith of Islam as a whole. The heritage of the symbols, however, really is based on links to the Ottoman empire, not the faith of Islam itself.
Even thousands of years ago, humans drew pictures to track the changes of the Moon. Later, people used their observations of the Moon to create calendars.
Today, we study the Moon using telescopes and spacecraft. For example, NASA's Lunar Reconnaissance Orbiter has been circling the Moon and sending back measurements since 2009.
The Moon is the only other planetary body that humans have visited. On July 20, 1969, NASA astronauts Neil Armstrong and Buzz Aldrin were the first people to set foot on the dusty surface of the Moon. Ten other American astronauts followed. They collected hundreds of pounds of lunar soil and rock samples, conducted experiments and installed equipment for follow-up measurements.
Astronaut Buzz Aldrin set up several scientific experiments while on the surface of the Moon during the historic Apollo 11 mission. You can see the lunar module, “Eagle,” in the background. Credit: NASA
Humans were only able to make that small step after several other space firsts happened. In 1957 the first artificial satellite, Sputnik 1, was launched into space by Russia. The United States launched several satellites of their own afterward. Both countries hoped to be the first to send a human into space.
It wasn’t until 1961 that a person went to space: On April 12, Russia’s Yuri Gagarin became the first. Less than a month later the United States’ Alan Shepard became the first American in space. Following these milestones, President John F. Kennedy issued a challenge to the National Aeronautics and Space Administration (NASA) to put a human on the moon in 10 years or less.
NASA went to work. On July 16, 1969, the spacecraft Apollo 11 prepared to launch a crew of three astronauts into space … and the history books.
Early forays into space
The earliest forays into lunar exploration were a product of the ongoing Cold War, when the U.S. and Soviet Union sent uncrewed spacecraft to orbit and land on the moon.
The Soviets scored an early victory in January 1959, when Luna 1, a small Soviet sphere bristling with antennas, became the first spacecraft to escape Earth’s gravity and ultimately fly within about 4,000 miles of the moon’s surface. (Read more about early spaceflight.)
Later in 1959, Luna 2 became the first spacecraft to make contact with the moon's surface when it crashed in the Mare Imbrium basin near the Aristides, Archimedes, and Autolycus craters. That same year, a third Luna mission captured the first, blurry images of the far side of the moon—where the rugged highland terrain is markedly different from the smoother basins on the side closest to Earth.
Then, the U.S. got in the game with nine NASA Ranger spacecraft that launched between 1961 and 1965, and gave scientists the first close-up views of the moon’s surface. The Ranger missions were daring one-offs, with spacecraft engineered to streak toward the moon and capture as many images as possible before crashing onto its surface. By 1965, images from all the Ranger missions, particularly Ranger 9, had revealed greater detail about the moon’s rough terrain and the potential challenges of finding a smooth landing site for humans.
In 1966, the Soviet spacecraft Luna 9 became the first vehicle to land safely on the lunar surface. Stocked with scientific and communications equipment, the small spacecraft photographed a ground-level lunar panorama. Later that year, Luna 10 launched, becoming the first spacecraft to successfully orbit the moon.
NASA also landed a spacecraft on the moon’s surface that year with the first of its Surveyor space probes, which carried cameras to explore the moon's surface and soil samplers to analyze lunar rock and dirt. Over the two years that followed, NASA launched five Lunar Orbiter missions that were designed to circle the moon and chart its surface in preparation for the ultimate goal: landing astronauts on the surface. These orbiters photographed about 99 percent of the moon's surface, revealing potential landing sites and paving the way for a giant leap forward in space exploration.(See a map of all lunar landings.)
The Origin of the Moon
Two PSI senior scientists, Dr. William K. Hartmann and Dr. Donald R. Davis, were the first to suggest the leading modern hypothesis of the moon's origin, in a paper published in 1975 in the journal Icarus.
Painting copyright William K. Hartmann
The idea in a nutshell:
At the time Earth formed 4.5 billion years ago, other smaller planetary bodies were also growing. One of these hit earth late in Earth's growth process, blowing out rocky debris. A fraction of that debris went into orbit around the Earth and aggregated into the moon.
Why this is a good hypothesis:
- The Earth has a large iron core, but the moon does not. This is because Earth's iron had already drained into the core by the time the giant impact happened. Therefore, the debris blown out of both Earth and the impactor came from their iron-depleted, rocky mantles. The iron core of the impactor melted on impact and merged with the iron core of Earth, according to computer models.
- Earth has a mean density of 5.5 grams/cubic centimeter, but the moon has a density of only 3.3 g/cc. The reason is the same, that the moon lacks iron.
- The moon has exactly the same oxygen isotope composition as the Earth, whereas Mars rocks and meteorites from other parts of the solar system have different oxygen isotope compositions. This shows that the moon formed form material formed in Earth's neighborhood.
- If a theory about lunar origin calls for an evolutionary process, it has a hard time explaining why other planets do not have similar moons. (Only Pluto has a moon that is an appreciable fraction of its own size.) Our giant impact hypothesis had the advantage of invoking a stochastic catastrophic event that might happen only to one or two planets out of nine.
What were some earlier ideas?
- One early theory was that the moon is a sister world that formed in orbit around Earth as the Earth formed. This theory failed because it could not explain why the moon lacks iron.
- A second early idea was that the moon formed somewhere else in the solar system where there was little iron, and then was captured into orbit around Earth. This failed when lunar rocks showed the same isotope composition as the Earth.
- A third early idea was that early Earth spun so fast that it spun off the moon. This idea would produce a moon similar to Earth's mantle, but it failed when analysis of the total angular momentum and energy involved indicated that the present Earth-moon system could not form in this way.
Where did the theory come from?
Hartmann and Davis were familiar with the work done in the Soviet Union in the 1960's, on the aggregation of planets out of countless asteroid-like bodies called planetesimals. Much of this work was pioneered by a Russian astrophysicist named V. S. Safronov.
Picking up on Safronov's general ideas, Hartmann and Davis ran calculations of the rate of growth of the 2nd-largest, 3rd largest, etc., bodies in the general vicinity of Earth, as the Earth itself was growing. Just as the asteroid belt today has a largest asteroid (Ceres) at a 1000 km diameter, and several smaller bodies in the 300-500 km diameter range, the region of Earth's orbit would have had several bodies up to about half the size of the growing Earth. Our idea was that in the case of Earth (but not the other planets) the impact happened late enough, and in such a direction relative to Earth's rotation, that abundant enough middle material was thrown out to make a moon.
How did the theory develop?
After we first presented the theory in 1974 at a conference on satellites, Harvard researcher A. G. W. Cameron rose to say that he and William Ward were also working on the same idea, but coming at it from a different motivation -- the study of angular momentum in the system -- and that they had concluded the impacting body had to be roughly Mars size (a third or half the size of Earth). Our paper was published in 1975 (Hartmann and Davis, Icarus, 24, 504-505) Cameron and Ward published an abstract on this idea at the Lunar Science conference in 1976, two years after the PSI paper.
Five Hours After Impact, based on computer modeling by A. Cameron, W. Benz, J. Melosh, and others. Copyright William K. Hartmann
Some work was done by Thompson and Stevenson in 1983 about the formation of moonlets in the disk of debris that formed around Earth after the impact. However, in general the theory languished until 1984 when an international meeting was organized in Kona, Hawaii, about the origin of the moon. At that meeting, the giant impact hypothesis emerged as the leading hypothesis and has remained in that role ever since. Dr. Michael Drake, director of the University of Arizona's Planetary Science Department, recently described that meeting as perhaps the most successful in the history of planetary science.
A collection of papers from that meeting was published by the Lunar and Planetary Institute (Houston) in the 1986 book, Origin of the Moon, edited by PSI scientist William Hartmann, together with Geoffry Taylor and Roger Phillips. This book remains the prime reference on this subject. In the meantime, researchers such as Willy Benz, Jay Melosh, A. G. W. Cameron, and others have attempted computer models of the giant impact, to determine how much material would go into orbit. Some of these results have been used by Hartmann to make the paintings on this web page, attempting to show how the impact would have looked to a human observer (if humans had been around -- they didn't come along until 4.5 billion years later!)
In the 1990's, Dr. Robin Canup wrote a Ph.D. dissertation on the moon's origin and the giant impact hypothesis, which produced new modeling of the aggregation of the debris into moonlets, and eventually, into the moon itself. Dr. Canup is continuing the modeling of the lunar accretion process.
In 1997, Dr. Canup's work received a great deal of publicity by media news sources, some of whom mistakenly thought that the giant impact was a brand new idea. Canup's early work, presented in July 1997, suggested the debris from an impact might not make a moon, but only a swarm of moonlets. Her later work (fall 1997) led to more "success" in aggregating the debris into a single moon.
At PSI we have worked with several leading researchers to propose new work or the accretion mechanics using a variant of the PSI planet building model. But this work has not been funded.
Hartmann, W. K. and D. R. Davis 1975 Icarus, 24, 505.
Hartmann, W. K. 1997. A Brief History of the Moon. The Planetary Report. 17, 4-11.
Hartmann, W. K. and Ron Miller 1991. The History of Earth, (New York: Workman Publishing Co.)
First Digital Photo of a President
It was only in 2009 that a digital camera was used to photograph POTUS. Official photographer Pete Souza holds the honor with his portrait of Barack Obama. Taken with a Canon 5D Mark II and no flash, the image shows the shifting gears of technology that have reached the White House.
Before the Columbia command module entered the Earth's atmosphere, it separated itself from the service module. When the capsule reached 24,000 feet, three parachutes deployed to slow down the Columbia's descent.
At 12:50 p.m. EDT on July 24, the Columbia safely landed in the Pacific Ocean, southwest of Hawaii. They landed just 13 nautical miles from the U.S.S. Hornet that was scheduled to pick them up.
Once picked up, the three astronauts were immediately placed into quarantine for fears of possible moon germs. Three days after being retrieved, Armstrong, Aldrin, and Collins were transferred to a quarantine facility in Houston for further observation.
On August 10, 1969, 17 days after splashdown, the three astronauts were released from quarantine and able to return to their families.
The astronauts were treated like heroes on their return. They were met by President Nixon and given ticker-tape parades. These men had accomplished what men had only dared to dream for thousands of years—to walk on the moon.