Lunar Phase
Lunar Phase Calculator - moonpage
The primary key to understanding moon phases is to think about the whereabouts of the sun. After all, it’s the sun that’s illuminating and creating the day side of the moon. Moon phases depend on the sun. They depend on where the moon is with respect to the sun in space.
Another key to understanding moon phases is to remember that, like the sun and all the planets and stars, the moon rises in the east and sets in the west each and every day. It has to. The rising and setting of all celestial objects is due to Earth’s continuous spin beneath the sky.
Also, remember that the moon takes about a month (one “month”) to orbit the Earth. Although the moon rises in the east and sets in the west each day (due to Earth’s spin), it’s also moving on the sky’s dome each day due to its own motion in orbit around Earth. The moon’s orbital motion can be detected in front of the stars from one night to the next. It’s as though the moon is moving on the inside of a circle of 360 degrees. Thus the moon moves about 12 degrees each day.
And remember that the moon’s orbital motion is toward the east. Each day, as the moon moves another 12 degrees toward the east on the sky’s dome, Earth has to rotate a little longer to bring you around to where the moon is in space. Thus the moon rises, on average, about 50 minutes later each day. The later and later rising time of the moon causes our companion world to appear in a different part of the sky at each nightfall for about two weeks. Then, in the couple of weeks after full moon, you’ll find the moon rising later and later at night.
Bottom line: This post explains why the moon waxes and wanes in phase, and gives some keys to understanding moon phases. It also provides links to descriptions of the various phases of the moon.
A waxing crescent moon – sometimes called a young moon – is always seen in the west after sunset.
At this moon phase, the Earth, moon and sun are located nearly on a line in space. If they were more precisely on a line, as they are at new moon, we wouldn’t see the moon. The moon would travel across the sky during the day, lost in the sun’s glare.
Here’s more to look for in tonight’s sky
But a waxing crescent moon is far enough away from that Earth-sun line to be visible near the sun’s glare – that is, in the west after sunset. This moon phase is seen one day to several days after new moon. On these days, the moon rises one hour to several hours behind the sun and follows the sun across the sky during the day. When the sun sets, and the sky darkens, the moon pops into view in the western sky.
Note that a crescent moon has nothing to do with Earth’s shadow on the moon. The only time Earth’s shadow can fall on the moon is at full moon, during a lunar eclipse. There is a shadow on a crescent moon, but it’s the moon’s own shadow. Night on the moon happens on the part of the moon submerged in the moon’s own shadow. Likewise, night on Earth happens on the part of Earth submerged in Earth’s own shadow.
Because the waxing crescent moon is nearly on a line with the Earth and sun, its illuminated hemisphere – or day side – is facing mostly away from us. We see only a slender fraction of the day side: a crescent moon. Each evening, because the moon is moving eastward in orbit around Earth, the moon appears farther from the sunset glare. It is moving farther from the Earth-sun line in space. Each evening, as the moon’s orbital motion carries it away from the Earth-sun line, we see more of the moon’s day side. Thus the crescent in the west after sunset appears to wax, or grow fatter each evening.
You sometimes see a pale glow on the darkened portion (night side) of a crescent moon. This glow is due to light reflected from Earth’s day side. It’s called earthshine.
A Last quarter moon shows half of its lighted hemisphere – half of its day side – to Earth.
But we officially call this moon a quarter and not a half because it is one quarter of the way around in its orbit of Earth, as measured from one new moon to the next.
This moon appears half-lit to us, and half moon is a beloved name (although not an official one). Still, it’s good to recall that the illuminated portion of a Last quarter moon truly is just a quarter. On the night of Last quarter moon, we see half the moon’s day side, or a true quarter of the moon. Another lighted quarter of the moon shines just as brightly in the direction opposite Earth!
A Last quarter moon rises at noon and is high overhead at sunset. It sets around midnight.
Last quarter moon comes a week after new moon. Now, as seen from above, the moon in its orbit around Earth is at right angles to a line between the Earth and sun.
A waxing gibbous moon appears high in the east at sunset. It’s more than half-lighted, but less than full.
This moon phase comes between one and two weeks after new moon.
The moon has moved in its orbit so that it’s now relatively far from the sun in our sky.
A waxing gibbous moon rises during the hours between noon and sunset. It sets in the wee hours after midnight.
People sometimes see a waxing gibbous moon in the afternoon, shortly after moonrise, while it’s ascending in the east as the sun is descending in the west. It’s easy to see a waxing gibbous moon in the daytime because, at this phase of the moon, a large fraction of the moon’s day side is facing our way. Thus a waxing gibbous moon is more noticeable in the sky than a crescent moon, with only a slim fraction of the lunar day side visible. Also, a waxing gibbous moon is far from the sun on the sky’s dome, so the sun’s glare isn’t hiding it from view.
Any moon that appears more than half lighted but less than full is called a gibbous moon. The word gibbous comes from a root word that means hump-backed. You can see the hump-backed shape of the waxing gibbous moon.
At full moon, we are seeing all of the moon’s day side.
And that’s what makes a full moon look full. At full moon, the moon and sun are on a line, with Earth in between. It’s as though Earth is the fulcrum of a seesaw, and the moon and sun are sitting on either end of the seesaw. So as the sun sets in the west, the full moon rises. When the sun is below our feet at midnight, the full moon is highest in the sky. When the sun rises again at dawn, the full moon is setting.
If there is a lunar eclipse, it must happen at full moon.
It’s only at the full moon phase that Earth’s shadow, extending opposite the sun, can fall on the moon’s face.
In many ways, a full moon is the opposite of a new moon. At both the new and full phases, the moon is on a line with the Earth and sun. At new moon, the moon is in the middle position along the line. At full moon, Earth is in the middle. full moon always comes about two weeks after new moon, when the moon is midway around in its orbit of Earth, as measured from one new moon to the next.
Bottom line: A full moon looks full because it’s opposite Earth from the sun, showing us its fully lighted hemisphere or day side.
A waning gibbous moon sails over the eastern horizon in the hours between sunset and midnight.
The moon is past full now. Once again, it appears less than full but more than half lighted.
What can I say about a waning gibbous moon? Only that it can surprise you if you happen to be out late in the evening. It rises eerily some hours after sunset, glowing red like a full moon when it’s near the horizon. Sometimes it looks like a misshapen clone of a full moon.
Because it comes up late at night, the waning gibbous moon prompts people to start asking, “Where is the moon? I looked for it last night and couldn’t find it.”
The waning gibbous moon also initiates a rash of questions about seeing the moon during the day. If it rises late at night, you know the waning gibbous moon must set after sunrise.
In fact, in the few days after full moon, you’ll often see the waning gibbous moon in the west in early morning, floating against the pale blue sky.
A Last quarter moon looks half-illuminated. It rises around midnight, appears at its highest in the sky at dawn, and sets around noon.
Last quarter moon comes about three weeks after new moon. Now, as seen from above, the moon in its orbit around Earth is at right angles to a line between the Earth and sun. The moon is now three-quarters of the way around in its orbit of Earth, as measured from one new moon to the next.
It’s very cool to see the Last quarter moon just after it rises, around midnight. The image at the top of this post illustrates it well. Just after the Last quarter moon rises, its lighted portion is always facing downwards, more or less toward the horizon. Why? Because you’re on the midnight portion of Earth – with the sun below your feet – just as it’s below this moon.
After the Last quarter phase, the moon will begin edging noticeably closer to the sun again on the sky’s dome. Fewer people notice the moon during the day from about Last quarter on, because the sun’s glare begins to dominate the moon.
A Last quarter moon can be used as a guidepost to Earth’s direction of motion in orbit around the sun. In other words, when you look at a Last quarter moon high in the predawn sky, you’re gazing out approximately along the path of Earth’s orbit, in a forward direction. The moon is moving in orbit around the sun with the Earth. But, if we could somehow anchor the moon in space . . . tie it down, keep it still . . . Earth’s orbital speed of 18 miles per second would carry us across the space between us and the moon in only a few hours.
As the moon orbits Earth, it changes phase in an orderly way. Follow these links to understand the various phases of the moon.
A waning crescent moon is sometimes called an old moon. It’s seen in the east before dawn.
Now the moon has moved nearly entirely around in its orbit of Earth, as measured from one new moon to the next.
Because the moon is nearly on a line with the Earth and sun again, the day hemisphere of the moon is facing mostly away from us once more. We see only a slender fraction of the moon’s day side: a crescent moon.
Each morning before dawn, because the moon is moving eastward in orbit around Earth, the moon appears closer to the sunrise glare. We see less and less of the moon’s day side, and thus the crescent in the east before dawn appears thinner each day.
The moon, as always, is rising in the east day after day. But most people won’t see this moon phase unless they get up early. When the sun comes up, and the sky grows brighter, the waning crescent moon fades. Now the moon is so near the Earth/sun line that the sun’s glare is drowning this slim moon from view.
Still, the waning crescent is up there, nearly all day long, moving ahead of the sun across the sky’s dome. It sets in the west several hours or less before sunset.
We can’t see the new moon from Earth, except during the stirring moments of a solar eclipse. Then the moon passes in front of the sun, and the night portion of the moon becomes visible to us, surrounded by the sun’s fiery corona.
Once each month, the moon comes all the way around in its orbit so that it is more or less between us and the sun. If the moon always passed directly between the sun and Earth at new moon, a solar eclipse would take place every month. But that doesn’t happen every month. Instead, in most months, the moon passes above or below the sun as seen from our earthly vantage point.
Young moon, visible a day or two after the new moon phase.
A young moon is seen in the west after sunset. It's a waxing crescent moon.
On the day of new moon, the moon rises when the sun rises. It sets when the sun sets. It crosses the sky with the sun during the day.
That’s why we can’t see the new moon in the sky. It is too close to the sun’s glare to be visible. Plus its lighted hemisphere is facing away from us.
Then a day or two later, the moon reappears, in the west after sunset. Then it’s a slim waxing crescent visible only briefly after sunset – what some call a young moon.
Each new lunar cycle is measured beginning at each new moon. Astronomers call one lunar cycle a lunation.
As the moon orbits Earth, it changes phase in an orderly way. Follow these links to understand the various phases of the moon.
From
http://earthsky.org/moon-phases/
|
| Phase |
Northern Hemisphere |
Southern Hemisphere |
Visibility |
Mid-phase
standard time |
Average
moonrise time |
Average
moonset time |
| New moon |
Disk completely in Sun's shadow (lit by earthshine only) |
Invisible (too close to Sun) |
Noon |
6:00 am |
6:00 pm |
| Waxing crescent |
Right side, 1–49% lit disc |
Left side, 1–49% lit disc |
Late morning to post-dusk |
3 pm |
9:00 am |
9:00 pm |
| First quarter |
Right side, 50%-lit disc |
Left side, 50%-lit disc |
Afternoon and early evening |
6 pm |
Noon |
Midnight |
| Waxing gibbous |
Right side, 51–99% lit disc |
Left side, 51–99% lit disc |
Late afternoon and most of night |
9 pm |
3:00 pm |
3:00 am |
| Full moon |
Completely illuminated disc |
Sunset to sunrise (all night) |
Midnight |
6:00 pm |
6:00 am |
| Waning gibbous |
Left side, 51–99% lit disc |
Right side, 51–99% lit disc |
Most of night and early morning |
3 am |
9:00 pm |
9:00 am |
| Last (or Third) quarter |
Left side, 50%-lit disc |
Right side, 50%-lit disc |
Late night and morning |
6 am |
Midnight |
Noon |
| Waning crescent |
Left side, 49-1% lit disc
Diminishing to the Moon's last visible crescent
|
Right 1–49% lit disc |
Pre-dawn to early afternoon |
9 am |
3:00 am |
3:00 pm |
When the Sun and Moon are aligned on the same side of the Earth, the moon is "new", and the side of the Moon facing Earth is not illuminated by the Sun. As the moon waxes (the amount of illuminated surface as seen from Earth is increasing), the lunar phases progress through new moon, crescent moon, first-quarter moon, gibbous moon, and full moon. The moon is then said to wane as it passes through the gibbous moon, last-quarter moon, crescent moon and back to new moon. The terms "old moon" and "new moon" are interchangeable, although new moon is more common. Half moon is often used to mean the first- and last-quarter moons, while the term 'quarter' refers to the extent of the moon's cycle around the Earth, not its shape.
When a sphere is illuminated on one hemisphere and viewed from a different angle, the portion of the illuminated area that is visible will have a two-dimensional shape defined by the intersection of an ellipse and circle (where the major axis of the ellipse coincides with a diameter of the circle). If the half-ellipse is convex with respect to the half-circle, then the shape will be gibbous (bulging outwards, Origin: 1350–1400; Middle English < Latin gibbosus humped, equivalent to gibb ( a ) hump + -osus -ous[1] ), whereas if the half-ellipse is concave with respect to the half-circle, then the shape will be a crescent. When a crescent Moon occurs, the phenomenon of Earthshine may be apparent, where the night side of the Moon faintly reflects light from the Earth.
In the northern hemisphere, if the left side of the Moon is dark then the light part is growing, and the Moon is referred to as waxing (moving toward a full moon). If the right side of the Moon is dark then the light part is shrinking, and the Moon is referred to as waning (past full and moving toward a new moon). Assuming that the viewer is in the northern hemisphere, the right portion of the Moon is the part that is always growing (i.e., if the right side is dark, the Moon is growing darker; if the right side is lit, the Moon is growing lighter). In the southern hemisphere the Moon is observed from a perspective inverted to that of the northern hemisphere, so the opposite sides appear to grow (wax) and shrink (wane).
The above descriptions of the lunar phases apply for observers at temperate or high latitudes on the Earth. Observers in tropical latitudes see the Moon with its terminator apparently horizontal during the morning and evening. The crescent Moon can open upward or downward, with the "horns" of the crescent pointing up or down, respectively. When the Sun appears above the Moon in the sky, the crescent opens downward; when the Moon is above the Sun, the crescent opens upward. The crescent Moon is most clearly and brightly visible when the Sun is below the horizon, which implies that the Moon must be above the Sun, and the crescent must open upward. This is therefore the orientation in which the crescent Moon is most often seen from the Earth's tropics. The waxing and waning crescents look very similar. The waxing crescent appears in the western sky in the evening, and the waning crescent in the east, in the morning.
When the Moon, as seen from Earth, is a narrow crescent, the Earth as seen from the Moon is almost fully lit by the Sun. Often, the part of the Moon that is not directly lit by the Sun is sufficiently brightly lit by light reflected from the Earth to be easily visible from Earth. This phenomenon is called "earthshine", and is sometimes picturesquely described as "the old moon in the new moon's arms".
Non-Western cultures may use a different number of Moon phases, for example traditional Hawaiian culture has a total of 30 different Moon phases.[2]
The average calendrical month, which is 1/12 of a year, is about 30.44 days, while the Moon's phase (synodic) cycle repeats on average every 29.53 days. Therefore, the timing of the Moon's phases shifts by an average of almost one day for each successive month. Photographing the Moon's phase every day for a month, starting in the evening after sunset, and repeating approximately 25 minutes later each successive day, ending in the morning before sunrise, would create a composite image like the example calendar from May 8, 2005, to June 6, 2005. There is no picture on May 20 since a picture would be taken before midnight on May 19, and after midnight on May 21. Similarly, on a calendar listing moon rise or set times, some days will appear to be skipped. When the Moon rises just before midnight one night it will rise just after midnight the next (so too with setting). The 'skipped day' is just a calendar artifact and not the Moon behaving strangely. The moon has a predictable orbit every month.
Calculating phase[edit]
Each of the 4 lunar phases lasts approximately 7 days (~7.4 days), but varies slightly due to lunar apogee and perigee.
The approximate age of the moon, and hence the approximate phase, can be calculated for any date by calculating the number of days since a known new moon (such as January 1, 1900 or August 11, 1999) and reducing this modulo 29.530588853 (the length of a synodic month). The difference between two dates can be calculated by subtracting the Julian Day Number of one from that of the other, or there are simpler formulae giving (for instance) the number of days since December 31, 1899. However, this calculation assumes a perfectly circular orbit and therefore may be incorrect by several hours (it also becomes less accurate the larger the difference between the required date and the reference date); it is accurate enough to use in a novelty clock application showing moon phase, but specialist usage taking account of lunar apogee and perigee requires a more elaborate calculation.[citation needed]
Effect of parallax[edit]
The Earth subtends an angle of about two degrees, when seen from the Moon. This means that an observer on Earth who sees the Moon when it is close to the eastern horizon sees it from an angle that is about two degrees different from the line of sight of an observer who sees the Moon on the western horizon. The Moon moves about 12 degrees around its orbit per day, so, if these observers were stationary, they would see the phases of the Moon at times that differ by about one-sixth of a day, or four hours. But in reality the observers are on the surface of the rotating Earth, so someone who sees the Moon on the eastern horizon at one moment sees it on the western horizon about 12 hours later. This adds an oscillation to the apparent progression of the lunar phases. They appear to occur more slowly when the Moon is high in the sky than when it is below the horizon. The Moon appears to move jerkily, and the phases do the same. The amplitude of this oscillation is never more than about four hours, which is a small fraction of a month. It does not have any obvious effect on the appearance of the Moon. However, it does affect accurate calculations of the times of lunar phases.
Misconceptions[edit]
The lunar phase depends on the Moon's position in orbit around the Earth and the Earth's position in orbit around the sun. This animation ( not to scale) looks down on Earth from the north pole of the ecliptic.
It might be expected that once every month, when the Moon passes between Earth and the Sun during a new moon, its shadow would fall on Earth causing a solar eclipse, but this does not happen every month. Nor is it true that during every full moon, the Earth's shadow falls on the Moon, causing a lunar eclipse. Solar and lunar eclipses are not observed every month because the plane of the Moon's orbit around the Earth is tilted by about five degrees with respect to the plane of Earth's orbit around the Sun (the plane of the ecliptic). Thus, when new and full moons occur, the Moon usually lies to the north or south of a direct line through the Earth and Sun. Although an eclipse can only occur when the Moon is either new (solar) or full (lunar), it must also be positioned very near the intersection of Earth's orbit plane about the Sun and the Moon's orbit plane about the Earth (that is, at one of its nodes). This happens about twice per year, and so there are between four and seven eclipses in a calendar year. Most of these events are quite insignificant; major eclipses of the Moon or Sun are less frequent.
See also[edit]
|
