Why is the moon so high in the sky tonight?

The first quarter half moon is almost directly overhead in the Los Angeles area on March 14th. I did some geometry on that, noting that the vernal equinox is approaching (March 20) with a full moon the next day -- very early Easter. The sun is over the Equator.

Are these things related? Maybe it's my imagination, but I don't recall the moon being quite as high in the sky as it is tonight.

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  • Dr Bob
    Lv 6
    1 decade ago
    Favorite Answer

    You're right that the moon was very high in the sky tonight around sunset.

    The maximum height of an astronomical body, in the course of the day, is determined by its declination (and also your latitude, but that's a constant). Declination is the celestial equivalent of latitude.

    The sun's highest declination in the course of a year is about 23.4 degrees. This occurs at the summer solstice, so that's why the sun gets very high in the sky at that time.

    The moon traverses the entire ecliptic about once a month, so every month its declination varies between about -23.4 degrees (at which time it will never get high in the sky) and +23.4 degrees (at which time it does get high). (These remarks are for northern-hemisphere observers.)

    In addition, the moon's orbit is inclined to the ecliptic by about 5 degrees, so it can reach a maximum declination of about +23.4+5.0 = 28.4. Sometimes its peak declination for the month is as high as 28.4, whereas other times it is as low as +23.4-5.0=18.4.

    The moon's declination tonight happens to be 27.6 degrees; so for northern-hemisphere observers, the moon gets nearly as high as it ever does. (Last night it was even slightly higher, with a declination of 27.8 degrees.)

    -- edit

    There is a normal cycle of "high" moons. The full moon is always close to opposite the sun; so when the sun is low in the sky in winter, the full moon gets very high in the sky. Right now, the moon is close to first quarter. The first-quarter moon is always high in March (at sunset). Similarly, the last-quarter moon is always high in September (at sunrise).

    Thus, there is nothing unusual about a first-quarter moon in March being high in the sky around sunset; but there is an extra 4-degree "bonus" right now because the moon lies that amount north of the ecliptic.

    -- edit

    This "bonus" of a few degrees is part of an 18.6-year cycle. Around 2006, the moon reached its most extreme declinations of the cycle; that is, its orbit took it to declinations about 5 degrees more extreme than those reached by the sun. If we look at the moon 9.3 years later (around 2015), the opposite is true; and the moon's declinations will be 5 degrees less extreme than those of the sun.

    In 2008, we are at the start of the downside of these extreme declinations. Next year, the first-quarter moon in spring will be about a degree lower in the sky; and you will not see the moon rise quite so high in the sky again until around the period 2022 through 2026.

    Wikipedia has an article that gives this phenomenon a name: "lunar standstill".

    http://en.wikipedia.org/wiki/Lunar_standstill

    -- edit

    "Lunar standstill" is a recent term. The standard astronomical term for this 18.6-year cycle is "regression of the line of nodes." The line of nodes is the line at which the orbit of the moon (with its 5-degree inclination) intersects the ecliptic. This line slowly rotates, and completes a full rotation after 18.6 years. (In general, the term "line of nodes" applies to the intersection of the orbits of any two objects.)

    Ancient people such as those who built Stonehenge knew of this cycle. Suppose you note the point on the horizon at which the moon rises, and do this every day over a year. (You can measure this point in degrees -- 0 for north, 45 northeast, 90 east, etc.) You can then note the most extreme locations for the year. If you do this year after year, you will discover the 18.6-year cycle.

    Example: Suppose you live at latitude 45 north. In 2006, the extreme azimuths of rising were 48 and 132 degrees, covering a span of 84 degrees over the horizon. Nine years later, in 2015, the extremes will be 64 and 117, covering a span of only 53 degrees. A civilization that makes such measurements over several decades will discover the 18.6-year periodicity between the most extreme moon risings.

    The times when the moon reaches a point very high in the sky, as you noticed, match the times when the moon rises very far to the north (such as the risings at azimuth 48 in 2006).

  • 1 decade ago

    So my knowledge of astronomy is very limited and I'm just shooting from the hip here, but here's my thinking:

    The moon orbits the earth nearly once per month; the earth revolves once every day; therefore, shouldn't the position of the moon in the sky change hourly?

  • Anonymous
    1 decade ago

    Greetings!

    The most simple explanation i can give on why the moon seems to be higher on some nights than others is that because its orbit changes with relation to the viewer.

  • 1 decade ago

    maybe..is..somoking..

    some..good..dodo

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