page_title: Lunar standstill
section_title: Detailed explanation
hierarchical_section_title: Lunar standstill / Detailed explanation
caption_attribution_description: Diagram showing the precession of nodes of the Earth-Moon system
context_page_description: A lunar standstill is the gradually varying range between the northern and the southern limits of the Moon's declination, or the lunistices, over the course of one-half a sidereal month, or 13.66 days. One major, or one minor, lunar standstill occurs every 18.6 years due to the precessional cycle of the lunar nodes at that rate.
  At a major lunar standstill, the Moon's range of declination, and consequently its range of azimuth at moonrise and moonset, reaches a maximum. As a result, viewed from the middle latitudes, the Moon's altitude at upper culmination changes in just two weeks – from highest to lowest above the horizon due north or south, depending on the observer's hemisphere. Similarly, its azimuth at moonrise changes from northeast to southeast and at moonset from northwest to southwest. Solar eclipses at ascending node occurs in March, solar eclipses at descending node occurs in September. Lunar eclipses at descending node occurs in March, lunar eclipses at ascending node occurs in September.
  This time appears to have had special significance for the Bronze Age societies who built the megalithic monuments in Britain and Ireland.
context_section_description: A more detailed explanation is best considered in terms of the paths of the Sun and Moon on the celestial sphere, as shown in the first diagram. This shows the abstract sphere surrounding the Earth at the center. The Earth is oriented so that its axis is vertical.
  The Sun is, by definition, always seen on the ecliptic (the Sun's apparent path across the sky) while Earth is tilted at an angle of e = 23.5° to the plane of that path and completes one orbit around the Sun in 365.25636 days, slightly longer than one year due to precession altering the direction of Earth's inclination.
  The Moon's orbit around Earth (shown dotted) is inclined at an angle of i = 5.14° relative to the ecliptic. The Moon completes one orbit around the Earth in 27.32166 days. The two points at which Moon crosses the ecliptic are known as its orbital nodes, shown as "N1" and "N2" (ascending node and descending node, respectively), and the line connecting them is known as the line of nodes. Due to precession of the Moon's orbital inclination, these crossing points, the nodes and the positions of eclipses, gradually shift around the ecliptic in a period of 18.59992 years.
  Looking at the diagram, note that when the Moon's line of nodes (N1 & N2) rotates a little more than shown, and aligns with Earth's equator, (from front to back, N1, Earth, and N2, seem to be the same dot), the Moon's orbit will reach its steepest angle with the Earth's equator, and in 9.3 years (from front to back, N2, Earth, N1 seem to be the same dot) it will be the shallowest: the 5.14° declination (tilt) of the Moon's orbit either adds to (major standstill) or subtracts from (minor standstill) the inclination of earth's rotation axis (23.439°).
  The effect of this on the declination of the Moon is shown in the second diagram. During the course of the nodal period, as the Moon orbits the Earth, its declination swings from –m° to +m°, where m is a number in the range (e – i) ≤ m ≤ (e + i). At a minor standstill (e.g., in 2015), its declination during the month varies from –(e – i) = –18.5° to +(e – i) = 18.5°. During a major standstill (e.g., in 2005-2006), the declination of the Moon varied during each month from about –(e + i) = –28.5° to +(e + i) = 28.5°.
  However, an additional subtlety further complicates the picture. The Sun's gravitational attraction on the Moon pulls it toward the plane of the ecliptic, causing a slight wobble of about 9 arcmin within a 6-month period. In 2006, the effect of this was that, although the 18.6-year maximum occurred in June, the maximum declination of the Moon was not in June but in September, as shown in the third diagram.
caption: Apparent paths of the Sun and Moon on the celestial sphere (angles exaggerated for clarity)
exif: {}