Rabbit in the Moon

The new moon of Thursday, February 3, was the second new moon after the winter solstice and therefore marked the Chinese New Year, beginning the Year of the Rabbit. The full moon of Friday morning, February 18, just followed the fifteenth day of the first Chinese lunar month, which is known as the Lantern Festival.

The start of this year, then, is an ideal time to look at the features on the moon and see if you can find the “moon rabbit.”  Of course, there is no rabbit in the moon, just dark splotches which some people believe look like a rabbit.  Understanding the rabbit in the moon, then, involves understanding why the moon has dark and light features.

Oceanus Procellarum is the large mare in the center and upper left of this image.
Visible in the upper right is another large mare, Imbrium, and below is the small
round Mare Humorum.

Dark and Light

The moon is not of uniform brightness because it is not of uniform elevation.  The brighter regions are called highlands because they are higher than the darker regions.  Because early lunar observers mistook these dark areas for earthly seas, they are called maria (singular mare; pronounced ‘mah-ree-a’ and ‘mah-ray’), from the Latin word for ‘sea.’   One mare which is much larger than the others has the name ‘Oceanus Procellarum,’ as an ocean is bigger than a mere sea.  Similar, smaller features bear the names lacus (‘lake’) or sinus (‘bay’).  These terms have persisted long after we realized that the Moon has no liquid water and no oceans, seas or lakes.

Lunar Prospector
Image courtesy of NASA

Ancient Basaltic Lava

Lunar mare are in fact ancient basaltic lava flows which filled basins of very large caters on the moon.  Evidence based on radiometric dating indicates that the maria formed between 3.15 and 4.2 billion years ago, with most of the lava flows occurring between 3.15 and 3.8 billion years ago.  This would mean their formation followed the Late Heavy Bombardment (4.1 to 3.8 billion years ago), when countless planetesimals collided with the inner planets of our solar system, forming many craters.  It appears that during this period, some of the larger impacts fractured the lunar regolith.  A few million years later, basaltic lava flowed into the resulting basins.

Why do all the moon’s maria face Earth?

The maria are not spread evenly across the moon’s surface, but instead are almost all on the near side of the moon, which always faces Earth.  The reason for this is a topic of active debate and research among lunar scientists.  Data from the Lunar Prospector mission indicates that under the lunar crust is a layer abnormally high in potassium (K), phosphorous (P), and rare earth elements (REE).  Further, this KREEP material is not spread evenly across the moon but is instead concentrated on the near side, specifically in the Oceanus Procellarum and Mare Imbrium basins.  As potassium and the rare earth elements uranium and thorium are heat-producing, their presence may have favored basaltic lava flows on the near side as opposed to the far side.

Naming Lunar Geography

The mare names we use today go back to Italian Jesuit astronomers Giovanni Battista Riccioli and Francesco Maria Grimaldi.  In 1651, Grimaldi prepared a map of the moon which Riccoli published in his Almagestum Novum.  Folklore associating the first quarter moon with calm weather and the last quarter moon with storms influenced Riccoli as he named the features Grimaldi had drawn.  The western limb of the moon, visible at first quarter, has seas of  Tranquility, Serenity and Fertility (Fecunditatis).  The eastern limb of the moon, visible at last quarter, has seas of Rain (Imbrium), Clouds (Nubium)  and Moisture (Humorum), as well as an Ocean of Storms (Oceanus Procellarum).  Riccoli was not the first to name features on the moon; Michael van Langren and Johannes Hevelius had used different sets of names.  However, when later lunar mapmakers, such as Johann Schröter, used Riccoli’s names, they became standard.  Incidentally, Riccoli also labeled the lunar highlands as terrae (‘lands’), but that nomenclature has not continued to this day.

Moon Legends

Many people around the world have tended to make various pictures out of the darker regions on the Moon’s surface.  The scientific term for our tendency to imagine familiar figures on the moon, or in clouds, on trees, etc., is pareidolia.  Perhaps you are familiar with the man in the moon.  His face consists of Mare Imbrium and Mare Serenitatis (eyes) along with Mare Nubium (mouth).

The Chinese, however, imagined a rabbit in the moon.  Mare Nectaris and Mare Fecunditatis form the tips of the rabbit’s ears, which come together at Mare Tranquillitatis.  Mare Serenitatis marks his head.  The large maria Oceanus Procellarum and Mare Imbrium form the bulk of his body, with Mare Vaopurm as his forelegs and Mare Nubium and Mare Humorum as his hind legs.

In an alternate image, the rabbit is facing the east (left) limb of the moon and is running instead of sitting.  In this view, the head becomes Oceanus Procellarum and the main part of the body Mare Imbrium.  The ears of the previous rabbit, mare Fecunditatis and Mare Nectaris, become the hind legs of this one.  Mare Nubium and Mare Humorun are now forelegs.  Mare Frigoris, a long ‘sea’ near the northern limb of the moon which did not figure into the previous rabbit, becomes a long ear of this one.

Photo edited by Zeimusu:
The rabbit stands by a cooking pot.
Based on the public domain moon image from
image:Luna_nearside.jpg and information on the web.

In Chinese folkore, this is the Jade Rabbit, making the elixir of life for the goddess of the moon Chang’e. In Japan and Korea, the moon rabbit makes rice cakes.  A Buddhist legend tells that the monkey, otter, jackal, and rabbit resolved to offer food to a stranger passing through the forest on the night of the full moon.  The rabbit, able to gather nothing but the grass he ate, offered his own body instead, and was rewarded by being placed in the moon.

Asian societies were not alone in imagining a rabbit in the moon.  In Aztec legend the god Tecciztecatl became the moon god after he hesitated to sacrifice himself in fire to become the sun god.  As punishment, the gods decided the moon would not be as bright as the sun. The Maya also associated a rabbit with their moon goddess.

Today, our Easter holiday is associated with bunnies.  That holiday bears a pre-Christian name which the Venerable Bede attributed to a goddess Eostre, who was associated with rabbits (among other symbols of life and fertility).  Perhaps finding the rabbit in the full moons of March and April can put you in the spirit of the Easter holiday.

The Great Cosmic Year

Our Milky Way Galaxy..
Creative Commons License photo credit: Sir Mervs

One of the biggest challenges in teaching astronomy to kids – or even to the general public – is that astronomy involves numbers so big as to be virtually meaningless. Consider the age of the universe, for example. Our best data indicate that the Big Bang, where space and time began, occurred about 13.7 billion years ago. As very few of us have seen 13.7 billion of anything before, how can we appreciate how long a time that is?

One way is to use a scale-model. Just as we use globes because the real Earth is too big to look at, we can ‘shrink’ the 13.7 billion year history of the universe into one year. Imagine a Great Cosmic Year, in which the Big Bang occurs at 12:00:00 am on January 1 and the present moment is 11:59:59.9999999 pm on December 31. On this time scale, each day represents (13.7 billion/365) years, or about 37.5 million years. Our best estimates for when the events listed below occurred are approximate; the dates listed may need to be adjusted slightly in the future.

Still, locating the events in the history of the universe, the Sun, and the Earth on this calendar can give us a better sense of how much time is involved.

January 1, midnight The Big Bang occurs.

January 13 The oldest known star in our galaxy (designated HE 1523-0901) forms.

‘HE’ here refers to the Hamburg/ESO (European Southern Observatory) survey, in which the star is catalogued. Being about 100 times too dim to be seen with the unaided eye, the star has no common name. It is in the constellation Libra.

Planet Earth (III)
Creative Commons License photo credit: Aaron Escobar

January 4-27 Re-ionization occurs.

We take for granted that the universe is transparent; that we can look through space and see galaxies, stars, and other planets. However, once hydrogen atoms formed in the early universe, this would have been impossible, as hydrogen atoms readily absorb photons (light particles). After the first billion years (corresponding to January 27 in the Great Cosmic Year), the hydrogen had been re-ionized. This happens when the electron in the hydrogen atom is too energetic too remain in orbit around the single proton which makes up the hydrogen nucleus. Newly formed stars and galaxies provided much of this energy.

April 14 First Sun-like stars (population I) appear.

Hydrogen and helium are so abundant in the universe that astronomers lump all other elements into a catch-all category called ‘metals.’ Astronomers divide stars into three categories based on their ‘metallicity,’ or how much stuff other than H or He they contain. This is important because those ‘metals’ ultimately make up solid things such as planet Earth, or you or me. Our Sun is only about 2% ‘metal.’

Stars of comparable metallicity are the youngest and are placed in population I. Some older stars in the distant halo of our galaxy are much less ‘metallic’ than our Sun, in some cases by a factor of 1,000 or 10,000; these are population II stars. Since all elements heavier than helium are formed in stars, astronomers speculate that the very first stars had virtually no metals, but such ‘population III’ stars have yet to be discovered.

It took about four billion years to make the first population I stars, bringing us to April 14 in our Great Cosmic Year.

Andromeda, again.
Creative Commons License photo credit: makelessnoise

May 23 The Milky Way’s galactic thin disc forms. This part of our galaxy includes our Sun.

August 31 Our solar system forms from a spinning cloud of dust.

The first population I stars to formed back on ‘April 14’ did not include our Sun. Astronomers recently discovered decay products of 60Fe, an isotope of iron that results from supernovae (exploding stars), in some meteorites. This suggests that a nearby supernova ejected this material into the dust cloud that became our solar system, making our sun at least a second generation population I star.

September 2 Earth begins to form.

Bad Moon Rising
Creative Commons License photo credit: makelessnoise

September 3 The Moon forms when a Mars-sized object called ‘Theia’ strikes Earth.

September 21 Earth begins to solidify.

This corresponds to the end of the Late Heavy Bombardment, a period of frequent impacts on all bodies in the inner solar system. Up to this point, consistent bombardment kept the Earth molten, with magma seas. With the end of the bombardment, Earth began to cool, solid rocks appeared, and Earth’s geologic history began.

September 29 Life begins on Earth.

October 12 The first continent (called Ur) appears on Earth.

November 2 Oxygen (O2) builds up in Earth’s atmosphere.

November 14 Eukaryotes (with distinct nuclei in the cell) exist on Earth.

November 27 Multicellular organisms exist.

December 5 The supercontinent Rodinia forms.

December 17 Cambrian explosion: earliest forms of most types (phyla) of animals appear.

December 20 First life on land

Of course, the real dinosaurs were bigger,
and not made of paper.
Creative Commons License
photo credit: kekremsi

December 25-29 Age of the dinosaurs

December 30 (morning) Chicxulub meteor impact helps cause extinction of about 3/4 of all life, including the dinosaurs.

The following events all occur on December 31:

9:17 am Drake passage completes the isolation of Antarctica; the continent freezes over.

7:30 pm Human ancestors diverge from chimpanzees.

9:57 pm Lucy lives in east Africa.

11:52 pm Homo sapiens sapiens exists.

11:59:14 pm Last Glacial Maximum (most recent Ice Age)

11:59:45pm Uruk, in Sumer, is one of the first cities on Earth.

Our existence as a species, compared to the whole universe, is about eight minutes out of a year. All of human civilization amounts to about 15 seconds. Once, I presented this calendar and was told that the smallness of our existence was an attack on religious faith. Perhaps, however, this need not be so. After all, an important virtue in most religious traditions is humility. This is not the denial of our talents and value, but the realization that we, with our goals, hopes, and dreams, are but one element of a much larger whole. As you reflect back on 2008 this holiday season, I invite you to reflect on the Great Cosmic Year. I find that the resulting wonderment and awe deepens my appreciation of the universe, and reminds me why I studied science in the first place.