Finding the Moon in the sky

It's very satisfying to be able to work out where you have to look to see the Moon on a particular day. But, to a casual observer, it can easily seem as if there isn't much rhyme or reason to when and where the Moon appears in the sky. Partly this is because, if the sky is overcast for several days in a row, we may miss seeing it altogether, and won't be able to see where each phase appears in the sky in relation to the others. In fact, the Moon moves across the sky like the Sun, rising in the east and setting in the west.

To make sense of the Moon's apparent motion you need to know a couple of things. The Moon orbits the Earth in a plane that is inclined at 5° to the ecliptic. Hence its apparent path across the celestial sphere, broadly speaking, is similar to that of the Sun. The Moon, however, takes just over 27 days to go around the celestial sphere once, whereas the Sun takes a whole year to do the same thing.

Since the Moon goes all the way around the celestial sphere in the course of a lunation, the points on the horizon at which it rises and sets during this period will swing between more or less the same extremes as those of the Sun do in the course of a year (see section 8.5 for the Sun's yearly motion). At the start of a lunation, the Moon lies directly between the Earth and the Sun. A young Moon therefore sets at almost the same point on the horizon as the Sun. But half a lunation later, the full Moon rises at a point on the horizon close to that at which the Sun will rise six months hence. Thus, in summer the daily path of the full Moon across the sky is similar to that taken by the Sun in winter: for observers in the northern hemisphere, it rises far to the south and remains close to the horizon as it crosses the sky from east to west. If you use the fact that the apparent motion of the Moon is linked to the Sun's position on the ecliptic, you won't find it too difficult to make a rough prediction of the path of the each of the Moon's key phases across the sky during each of the four seasons (see figure 9.14). And, of course, we always have an approximate idea of where the Sun is on the ecliptic since this determines the season. For example, in midsummer, the Sun is as far north of the celestial equator as it can get, and in autumn it crosses the celestial equator from north to south.

As an example of how you can work out the Moon's daily path across the sky in advance, consider a lunation that begins when the Sun is at, or very close to, the autumn equinox. Every day after the start of the lunation, the Moon moves eastward away from the Sun, more or less along the ecliptic. This half of the ecliptic lies south of the celestial equator, and so during the first half of this lunation, the waxing autumn Moon hugs the southern horizon. This makes it a difficult object to see, since it may be hidden from view behind buildings, trees or hills. During the second half of the lunation, however, the Moon moves into the northern celestial hemisphere, and so the waning autumn Moon is seen high in the sky. The situation is reversed

Figure 9.14 The apparent path of the full Moon across the sky at different seasons. During autumn and spring, the full Moon follows more or less the same path across the sky as the Sun. In winter, the Sun remains close to the horizon throughout the day, while the Moon rises high into the sky at night. During summer, the situation is reversed, and the full Moon is never far from the horizon, while the Sun climbs high into the sky.

Figure 9.14 The apparent path of the full Moon across the sky at different seasons. During autumn and spring, the full Moon follows more or less the same path across the sky as the Sun. In winter, the Sun remains close to the horizon throughout the day, while the Moon rises high into the sky at night. During summer, the situation is reversed, and the full Moon is never far from the horizon, while the Sun climbs high into the sky.

Sunrise

Spring

Summer

Autumn

Winter

A Sunset

Sunrise

Sunset

( | ( ^ Moons phases: crescent, first quarter, waxing gibbous, full moon

- Celestial equator

Figure 9.15 These diagrams show the approximate position of the Moon's phases during the first half of a lunation at sunrise and at sunset. 'A' marks the spring equinox. Notice that the sky at sunrise during spring is the same as the sky at sunset in autumn, and vice versa. The same is true for summer and autumn.

The Moon's daily path across the sky is always parallel to the celestial equator, whatever its phase.

for the lunation that starts when the Sun is at the spring equinox: a waxing Moon in spring always rises higher in the sky than a waning Moon. Observers in the southern hemisphere see these events the other way around: a waxing crescent is higher in the sky in autumn than in spring.

The season in which each key phase reaches its maximum or minimum altitude above the horizon, i.e. when it culminates, is given in the table below. In this table 'maximum' means that the Moon will culminate between 18.5° and 28.5° above the celestial equator, and 'minimum' means it will culminate between 18.5° and 28.5° below the celestial equator. 'Intermediate' means that the Moon will culminate between 5° above and 5° below the celestial equator. 5° is, of course, the inclination of the Moon's orbital plane to the ecliptic.

Altitude of key phases at culmination in different seasons

First quarter

Full Moon

Last quarter

Spring

Maximum

Intermediate

Minimum

Summer

Intermediate

Minimum

Intermediate

Autumn

Minimum

Intermediate

Maximum

Winter

Intermediate

Maximum

Intermediate

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