Does the right ascension angle between the direction of the Sun and the Moon increase or decrease?






Does the Right Ascension angle between the direction of the Sun and the Moon increasing, decreasing, or staying the same during the month?
I had this question for an assignment and it’s the only question I really don’t understand or know how to answer. Any help would be appreciated.


  1. The R.A. angle is just the angle between the Sun and the Moon, measured parallel to the equator. Mostly, the sun and moon move parallel to the equator — the sun is 23.5 degrees off, and the moon is between 18.3 and 28.6 degrees off. That’s more parallel than not parallel.
    So the question can be understood as asking, does the separation angle between the sun and moon stay the same, or increase, or decrease during the month.
    Now, if it increased all the time, what would happen when it got to 180 degrees? Or if it decreased all the time, what would happen when it got to -180 degrees? Once you go half way around a circle, you’re getting closer to where you started.
    And, if the angle stayed the same all the time, the separation would not change (at least, not in R.A.).

  2. During the course of about a month, the moon makes a full orbit of the earth, and its right ascension varies through the full circle. During almost half of the month, the moon’s right ascension is greater than that of the sun (while the moon’s phase is waning) and during another nearly half, the moon’s right ascension is less than that of the sun (while the moon’s phase is waxing). The rest of the time, the moon’s right ascension is equal to that of the sun (at the time of new moon), or opposite to the sun’s right ascension (180 degrees or 12 hours away from the sun (at the time of the full moon).
    The moon’s right ascension increases slower than the sun’s right ascension. You’ll have to draw out some sketches to see what I’m saying, perhaps, but it’s not rocket science ;-))

  3. RA is expressed in hours, minutes, seconds (from 0 to 24 hours).
    The RA values get bigger leftwards.
    If you mean with ‘RA angle’ the distance (in hours, minutes, seconds) between the RA of the Moon and the RA of the Sun, then that distance grows from New Moon to Full Moon (Moon leaving the Sun at the left side of the Sun), it decreases again from Full Moon to New Moon (Moon getting closer to the sun again, right of the Sun).
    The RA value of the Sun itself increases, making a full round in 1 year. The RA value of the Moon itself also increases, but in a month time. So the RA of the Moon really increases faster.

  4. Right ascension is measured in hours, minutes and seconds eastward from zero degrees Aries. The moon appears, on average, about 51 minutes farther east each day. So the moon’s RA angle is greater each day until it reaches 24 hours; then it suddenly drops to zero and starts increasing again.
    The sun’s RA angle begins at zero on the March equinox and increases about one degree per day. If you subtract one from the other, you have to decide what to do when one of them drops to zero. For example, if the RA of the sun is 23 hours, and the RA of the moon is 1 hour, the difference can be minus 22 hours or plus 2 hours. You need to ask the teacher to clarify what is meant by, “right ascension angle between the direction of the sun and the moon”. This is a non-standard term, so it’s no wonder you are confused.

  5. I might as well start off from where ‘morningf..’ left. He brought the ‘issue’ into focus.
    First of all it is necessary to introduce ‘Zodiac’ comprising of the 12 constellations called ‘signs’, which is the common highway for all planets of Solar family besides Sun & Moon, (celestial) Great Circle with an inclination of 23.45 deg cutting the (celestial) Equator (RA = 0 at every point) at ascending node that is traditionally known as ‘first point of Aries’ and is now near the junction of Aquarius & Pisces constellations. Zodiac is the reference for the following discussion and is the path of Sun (as viewed from Earth).
    Moon deviates within +/- 5deg 8′ 40″ from Zodiac. At two points it is exactly at the same RA as Sun’s path (reference or center line of Zodiac). This pair of points are the ascending & descending nodes. What’s more – this diametrically (on Zodiac) opposite pair (A & D nodes) are not fixed points on Zodiac. They go in a retrograde motion (From Aries to Pisces, Aquarius and so on), with a period of 18 2/3 years. Another point is – while the other paths display their elliptical properties, making ‘Apogees’ and ‘Perigees’, the nodal pair (intersecting points of Sun’s & Moon’s) are circular in that their movement is uniform (circular).
    In Hindu Astrology, wherein it becomes necessary to track the paths of all visible planets besides Sun & Moon, this pair of points (accorded the status of planets) is also tracked under the names of ‘Rahu’ & ‘Kaetu’.

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