Tag Archives: moon

Educator How-To: We’ll See You on the Dark (and Light and Far) Side of the Moon

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Guess what time it is? Yup, you guessed it. It’s time for another installment of my Educator How-Tos! This week, we’re going celestial with a phases of the moon demonstration.

The moon over HMNS' George Observatory.

The phases of the moon is an important concept that students must understand. After years of experience, we here at the Museum feel we have refined this activity to the point that it can be taught and understood easily.

Step One. Determine what your audience “thinks” they know. We need them to understand the following before we begin:

  • The moon orbits earth.
  • Sunlight reflects off the surface of the moon to create brightness.
  • The moon appears to change shape over time, and we call this change “the phases of the moon.”

Some misconceptions you may encounter:

  • The moon produces its own light, like the Sun. No, the moon reflects sunlight, and that is why it is illuminated.
  • Earth’s shadow explains the phases of the moon. This is an eclipse and is a completely different event.
  • The moon does not rotate. This is incorrect. For our purposes, the moon will not rotate in the demonstration, but the moon does indeed rotate, albeit more slowly than earth. The moon does not appear to rotate because we always see the same side. However, the reason we always see the same side is because it rotates exactly one time during each orbit around the earth.
  • People confuse the dark side of the moon and the far side of the moon. There is indeed a dark side, and a light side for that matter, but this is a cyclical event with alternating sides of the moon being dark and light at different times. The far side of the moon, however, is the side of the moon that we never see from our vantage point on earth.

Step Two. Ask your audience if they can name the phases of the moon. For our purposes today, we will only be considering the full moon, half moon (quarter moon), crescent moon, and new moon.

Special Note: The half moon is traditionally called the quarter moon, but I find it confusing, so I use the term half moon instead. You may handle this challenge in a way that works best for you. Both terms are correct.

Step Three. Explain this key concept before you begin the demonstration: There is always half of the moon that is in darkness and half that is in light at all times — regardless of the portion of the lighted moon that is visible to us on Earth. This can easily be explained by calling the dark side “nighttime” on the moon, and the light side “daytime.” Just as on earth, when one side is experiencing daytime, the other side is always experiencing night time.

Demonstration. Now you will demonstrate this concept using a bright light source (an overhead projector works well), a blow up Moon (or another light-colored ball that is large) and a volunteer.

The key concepts determined in Step One (light reflects off the moon as the moon orbits the Earth and the moon appears to change shape) are modeled in this demonstration. Use the following steps to demonstrate moon phases:

  • Pick a volunteer from the audience.
  • Tell the class that the volunteer is going to represent our perspective of the “moon” from different angles. They should envision viewing the “moon” through the volunteer’s eyes.
  • Have the volunteer stand directly to the left of the light source (which is representative of the Sun).
  • The instructor, holding the “moon”, should stand directly in front of the “sun” (about four to six feet away). Now that everyone is in the proper place, you may begin.
  • The volunteer, looking directly at the “moon” should describe the amount of light (or darkness) they observe. From this perspective the entire visible portion of the “moon” should be in the light. This is representative of a full moon. When one observes a full moon they are viewing the entire half of the Moon that is in full daylight.
  • Have the volunteer move 90 degrees to the left. Again ask them to describe what portion of the visible “moon” is in the light (or dark). From this new perspective, half of the visible portion of the “moon” should be illuminated. This is a half moon (quarter moon). When one observes a half moon they are only seeing half of the moon that is in actual daylight — the rest of the illuminated portion is not visible from an earthbound perspective.
  • The volunteer should move 45 degrees to the left. Again, ask them to describe what portion of the “moon” is in the light (or dark). From this new perspective 1/4 of the visible portion of the “moon” should be illuminated. This is what is called a crescent moon. When one observes a crescent moon, they are only viewing ¼ of the Moon that is in actual daylight — the rest is not visible from an earthbound perspective.
  • The volunteer should now move an additional 45 degrees to the left. The volunteer will now be standing opposite the instructor and the “moon”. Again, ask them to describe what portion of the “moon” is in the light (or dark). From this new perspective, none of the visible portion of the “moon” should be illuminated. This is what is called a new moon. When one observes a new moon, they observing the half that is in “nighttime,” but remember — the daytime side is still on the other side.
phases-of-the-moon

Viewpoints & Phases of the Moon

Key Point Summary:

  • The moon is always half daytime and half nighttime.
  • As the moon orbits the Earth, our perspective changes and we see different amounts of light and dark area.
  • The appearance of the change in shape is just a change in the amount of lighted side visible from Earth.

Go Stargazing! August Edition

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Saturn is the only planet visible to the naked eye at night this August.  Face southwest at dusk and you’ll see Saturn near a star of similar brightness—Spica in Virgo.  Saturn is a bit to the right of Spica as you face southwest.   The ringed planet remains well placed for evening viewing and remains in the night sky until late September 2011.

Hubble Images Suggest Rogue Asteroid Smacked Jupiter
Creative Commons License photo credit: NASA Goddard Photo and Video

Mars and Jupiter are in the pre-dawn sky.  Jupiter, set against a background of very dim stars, dominates the southeastern pre-dawn sky and is due south at dawn by the end of the month.  Mars is dimmer and much lower in the east northeast.  It has fully emerged from the sun’s glare and will brighten slightly each morning. Venus is now out of sight.  Superior conjunction (alignment on the far side of the sun) is on August 16.

The Big Dipper is to the left of the North Star, with its handle pointing up.  From that handle, you can ‘arc to Arcturus’ and then ‘speed on to Spica’; those stars are in the west and southwest at dusk.  Antares, brightest star of Scorpius, the Scorpion, is in the south, with the ‘teapot’ of Sagittarius to its left.  Meanwhile, the Summer Triangle is approaching the zenith.  The stars of summer now dominate the evening sky.  In late evening, you can watch the Great Square of Pegasus rise in the east.

Moon Phases in August 2011:

1st Quarter                     August 6, 6:08 a.m.

Full Moon                       August 13, 1:57 p.m.

Last Quarter                  August 21, 4:56 a.m.

New Moon                      August 28, 10:03 p.m.

The annual Perseid Meteor Shower peaks this year on Saturday morning, August 13.  Unfortunately, the moon (full on the 13th) hides all but the very brightest meteors and thus spoils the show.  If you want to see just how many Perseids can outshine the moonlight, the best hours are from roughly 2 a.m. to dawn.

Go Stargazing: April Edition

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Saturn dominates April 2011 skies because yesterday, on April 3, the Earth passed between the sun and Saturn.  This alignment, called opposition, puts Saturn in the sky all night long; it rises in the east at dusk and sets in the west at dawn.

Venus’ apparition as a dazzling morning star is coming to an end.  It is getting lower and lower in the sky each morning as the angle between the solar system plane and the horizon gets shallower.  Face southeast at dawn, and you can’t miss it of you have a clear horizon.

Jupiter is directly behind the sun from our perspective on April 6 and therefore invisible all month.

Mars also remains lost in the sun’s glare all month.

A swath of brilliant winter stars continues to dominate evening skies.  These stars are now high in the west at dusk and set in late evening.  Orion, the Hunter, is in the southwest as April begins.  His two dogs, represented by Sirius and Procyon, are to his left.  Beside Orion in the west is Taurus, the Bull with Aldebaran as its eye. Gemini, the Twins, are above Orion.  The Big Dipper is to the upper right of the North Star, with its handle pointing down and to the right.  From that handle, you can ‘arc to Arcturus’ and then ‘speed on to Spica’; those stars are low in the east at dusk.  Leo, the Lion, passes almost overhead in late evening.

Below Sirius, just above the southern horizon in late twilight, is a star second only to Sirius in brightness.  This is Canopus, which marks the keel (bottom) of the legendary ship Argo Navis.  Canopus is so far south, in fact, that most Americans never see it.  From the Gulf Coast, however, Canopus does rise.  In early April, you can still see it in the evening just after dusk.

Lune
Creative Commons License photo credit: ComputerHotline

Moon Phases in April 2011:

New Moon                      April 3, 9:32 a.m.

1st Quarter                     April 11, 7:05 a.m.

Full Moon                       April 17, 9:43 p.m.

Last Quarter                  April 24, 9:46 p.m.

Sunday, April 24, is the first Sunday after the first Full Moon of spring.  Therefore, this is Easter Sunday.  This happens to be the second latest possible date for Easter.  Easter will fall on April 25, the absolute latest date, in 2038.

Rabbit in the Moon

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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.