Inclination extremes Every 18.6 years, the angle between the Moon's orbit and Earth's
equator reaches a maximum of 28°36′, the sum of Earth's
equatorial tilt (23°27′) and the Moon's
orbital inclination (5°09′) to the
ecliptic. This is called
major lunar standstill. Around this time, the Moon's
declination will vary from −28°36′ to +28°36′. Conversely, 9.3 years later, the angle between the Moon's orbit and Earth's equator reaches its minimum of 18°20′. This is called a
minor lunar standstill. The last lunar standstill was a minor standstill in October 2015. At that time the descending node was lined up with the equinox (the point in the sky having
right ascension zero and
declination zero). The nodes are moving west by about 19° per year. The Sun crosses a given node about 20 days earlier each year. When the inclination of the Moon's orbit to the Earth's equator is at its minimum of 18°20′, the centre of the Moon's disk will be above the
horizon every day from latitudes less than 70°43' (90° − 18°20' – 57' parallax) north or south. When the inclination is at its maximum of 28°36', the centre of the Moon's disk will be above the horizon every day only from latitudes less than 60°27' (90° − 28°36' – 57' parallax) north or south. At
higher latitudes, there will be a period of at least one day each month when the Moon does not rise, but there will also be a period of at least one day each month when the Moon does not set. This is similar to the
seasonal behaviour of the Sun, but with a period of 27.2 days instead of 365 days. Note that a point on the Moon can actually be visible when it is about 34
arc minutes below the horizon, due to
atmospheric refraction. Because of the inclination of the Moon's orbit with respect to the Earth's equator, the Moon is above the horizon at the
North and
South Pole for almost two weeks every month, even though the Sun is below the horizon for six months at a time. The period from moonrise to moonrise at the poles is a
tropical month, about 27.3 days, quite close to the sidereal period. When the Sun is the furthest below the horizon (
winter solstice), the Moon will be full when it is at its highest point. When the Moon is in
Gemini it will be above the horizon at the North Pole, and when it is in
Sagittarius it will be up at the South Pole. The Moon's light is used by
zooplankton in the Arctic when the Sun is below the horizon for months and must have been helpful to the animals that lived in Arctic and Antarctic regions when the climate was warmer.
Declination extremes The Moon's orbit is inclined about 5.14° to the
ecliptic; hence, the Moon can be up to about 5° north or south of the ecliptic. The ecliptic is inclined about 23.44° to the
celestial equator, whose plane is
perpendicular to the rotational axis of Earth. As a result, once during the 18.6-year nodal period (when the ascending node of the Moon's orbit coincides with the
vernal equinox), the Moon's
declination reaches a maximum and minimum (northern and southern extremes): about 28.6° from the celestial equator. Therefore, the moonrise or moonset
azimuth has its northern- and southernmost points on the horizon; the Moon at
culmination has its lowest and highest
altitude (when the body transits the
meridian); and first sightings of the
new moon potentially have their latest times. Furthermore,
occultations by the Moon of the
Pleiades star cluster, which is over 4° north of the ecliptic, occur during a comparatively brief period once every nodal period. ==Effect on tides==