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Message: <blockquote data-quote="Mystery Man" data-source="post: 1922342" data-attributes="member: 13273">Well if it drifts all over the place it's not much of an axis now is it?!? <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f642.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=":)" title="Smile    :)"  data-smilie="1"data-shortname=":)" />    From the Encyclopedia Brittanica.... edit: I removed the links to figures and tables.    Principal characteristics of the Earth-Moon system      The movement of the Earth-Moon system is illustrated in Figure 1, and the Table lists some salient characteristics. The two bodies orbit each other about their centre of mass—called the barycentre—a point inside the Earth about 4,700 kilometres from its centre. The distance between the centres of mass of the Earth and the Moon varies rather widely due to the combined gravity of the Earth, the Sun, and the planets. For example, during the period 1969–2000, apogee (the greatest distance) varies from 404,063 to 406,711 kilometres, while perigee (the least distance) varies from 356,517 to 370,354 kilometres. Tidal interactions have braked the Moon's spin so that presently the same side always faces the Earth. As discovered by Gian D. Cassini in 1692, the Moon's spin axis precesses with its orbital plane; i.e., its orientation changes slowly over time, tracing out a circular path. The orbital geometry gives rise to the Moon's phases and to the phenomena of eclipses. When the Moon's orbital line of nodes (the points at which the Moon crosses the ecliptic plane) coincides with the Earth-Sun direction and the Moon is near a node, a solar or lunar eclipse can occur. These events happen in groups, sometimes called the Saros cycle, about every 18.6 years—the period of precession of the lunar orbital plane.   In accord with Kepler's laws, the eccentricity of the Moon's orbit results in faster and slower motion along the orbit. Combined with the Moon's constant spin rate, this gives rise to an apparent libration, enabling more than half of the lunar surface to become visible. In addition to this apparent motion, the Moon actually does rock slightly to and fro in both longitude and latitude, and the observer's vantage point moves with the rotation of the Earth. As a result, more than 59 percent of the lunar surface can be seen at one time or another from the Earth. The orbital eccentricity also affects eclipses of the Sun. If a solar eclipse occurs when the Moon is near perigee, observers along the Moon's umbral (dark, inner) shadow path see a total eclipse. If the Moon is near apogee, it does not quite cover the Sun; the resulting eclipse is annular, and observers can see a thin ring of the solar disk.   The Moon and the Earth presently orbit the barycentre in 27.322 days, the sidereal month. Because the whole system is moving around the Sun once per year, the angle of illumination changes about one degree per day, so that the time from one full Moon to the next is 29.531 days, the synodic month. These periods are slowly changing with time owing to tidal interactions. Tidal friction slows the Earth's rotation, but the angular momentum of the Earth-Moon system remains constant. Consequently, the Moon is slowly receding from the Earth, with the result that the month and the day are both getting longer. Extending this relationship back into the past, both periods must have been significantly shorter hundreds of millions of years ago, and this hypothesis is confirmed by measuring the diurnal and tide-related growth rings of fossil corals.     Because the Moon's spin axis is inclined only 1 1/2° from the normal to the ecliptic (see Figure 1), the Moon has no seasons. Sunlight is always nearly horizontal at the lunar poles, resulting in permanently cold and dark environments.</blockquote>

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