Spring will officially arrive on March 20 at 1:26 p.m. EST, when the Sun will shine directly down on the equator. Our four seasons are the by-product of the 23-½-degree tilt of the Earth's axis and during the course of a year we see the Sun cycle between declination 23-½ degrees north and 23-½ degrees south as it circles around the sky along the ecliptic.
The Sun's changing declination is what determines whether the Sun's rays strike us at a low angle or more directly. (Declination on a celestial globe is analogous to latitude on a terrestrial globe. A star with a declination of +30 degrees would appear to pass directly overhead as seen from 30 degrees north latitude).
The changing declination also defines the daily arc that the Sun takes across the sky, thereby accounting for the length of daylight.
Similarly, the Moon too goes through these up and down motions, but cycles through the entire sky much more quickly: once every month. And, in fact, the Moon can range even farther to the north and south than the Sun because its orbit is inclined 5.1 degrees to the ecliptic.
In special years this tilt adds to the ecliptic's own inclination and as a result the Moon can attain unusually high or low altitudes in our sky.
Such is the case in 2006.
The Moon's nodes-those two points where the Moon's orbit intersects the ecliptic-are not stationary, but move around the ecliptic. This happens because the Moon's orbit slowly "wobbles." This wobbling effect is due primarily to the Sun's gravitational pull on the Earth-Moon system and is referred to as the regression of the nodes. Indeed, the nodes advance very slowly westward ("regress") each month, completing one full circuit of the sky in 18.6 years.
In 2006, this shift will carry the Moon's ascending node through the position of the March equinox (which is located in the constellation Pisces). June 19 will be the exact date when the ascending node crosses the celestial equator. As a result, the Moon will end up being positioned farthest to the north and south of the ecliptic around the border of Taurus and Gemini, and on the other side of the sky in Sagittarius, where (respectively) the ecliptic itself is oriented farthest north and south.
So each month in 2006, when the Moon is passing near the Taurus/Gemini border it will appear to ride unusually high in the sky as it crosses the meridian. Conversely, when the Moon is moving through Sagittarius it will run unusually low. In the table below, are the dates when the Moon will passing through these areas of the sky.
Highest and lowest Moons in 2006:
Taurus/Gemini ("Runs High") | Sagittarius ("Runs Low") |
January 10, 11, 12 | January 25, 26, 27 |
February 7, 8, 9 | February 22, 23, 24 |
March 6FQ , 7, 8 | March 21, 22LQ, 23 |
April 2, 3, 4, 30 | April 18, 19, 20 |
May 1, 2, 27, 28, 29 | May 15, 16, 17 |
June 24, 25NM, 26 | June 11FM,12,13 |
July 21, 22, 23 | July 8, 9, 10 |
August 17, 18, 19 | August 4, 5, 6 |
September 13,14LQ,15 | September 1, 2, 3, 28, 29, 30FQ |
October 11, 12, 13 | October 25, 26, 27 |
November 7, 8, 9 | November 22,23,24 |
December 5, 6, 7 | December 19,20NM, 21 |
NM = New Moon; FQ = First Quarter; FM = Full Moon; LQ= Last Quarter
The Moon's greatest southerly declination, -28.72 degrees, will occur this week, on Wednesday, March 22, coinciding with the Last Quarter Moon. Just step outside about 10 or 15 minutes before sunrise on that morning and face south. You'll see the Moon hanging as low to the horizon as it can get while it crosses the meridian.
A somewhat similar circumstance will accompany the Full Moon on the night of June 11-12 (due south around 1:30 a.m. local daylight time) and the First Quarter Moon of September 30 (due south about 45 minutes after sunset).
For those in the Southern Hemisphere, the situation will be reversed: For a person in Perth, Australia, for example, the March 22 Moon will be "high" and the one in September will be "low."
In contrast, less than half a year from now, on September 14, the Moon-again at Last Quarter-will soar to its farthest declination north, +28.72 degrees. It will cross the southern meridian about 5 or 10 minutes after sunrise and even to a casual viewer that morning, its location will appear unusual. It will climb so very high up in the sky that from central Florida and southernmost Texas it will be seen directly overhead. And for watchers in Miami or Brownsville the Moon will actually go north of the zenith!
Basic Sky Guides
- Full Moon Fever
- Astrophotography 101
- Sky Calendar & Moon Phases
- 10 Steps to Rewarding Stargazing
- Understanding the Ecliptic and the Zodiac
- False Dawn: All about the Zodiacal Light
- Reading Weather in the Sun, Moon and Stars
- How and Why the Night Sky Changes with the Seasons
- Night Sky Main Page: More Skywatching News & Features
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.
| DEFINITIONS |
1 AU, or astronomical unit, is the distance from the Sun to Earth, or about 93 million miles. Magnitude is the standard by which astronomers measure the apparent brightness of objects that appear in the sky. The lower the number, the brighter the object. The brightest stars in the sky are categorized as zero or first magnitude. Negative magnitudes are reserved for the most brilliant objects: the brightest star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest stars visible under dark skies are around +6. Degrees measure apparent sizes of objects or distances in the sky, as seen from our vantage point. The Moon is one-half degree in width. The width of your fist held at arm's length is about 10 degrees. The distance from the horizon to the overhead point (called the zenith) is equal to 90 degrees. Declination is the angular distance measured in degrees, of a celestial body north or south of the celestial equator. If, for an example, a certain star is said to have a declination of +20 degrees, it is located 20 degrees north of the celestial equator. Declination is to a celestial globe as latitude is to a terrestrial globe. Arc seconds are sometimes used to define the measurement of a sky object's angular diameter. One degree is equal to 60 arc minutes. One arc minute is equal to 60 arc seconds. The Moon appears (on average), one half-degree across, or 30 arc minutes, or 1800 arc seconds. If the disk of Mars is 20 arc seconds across, we can also say that it is 1/90 the apparent width of the Moon (since 1800 divided by 20 equals 90). |
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