Lunar phase

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Lunar phases (or Phases of the Moon) are the result of looking at the illuminated half of the Moon from different viewing geometries; they are not caused by shadows of the Earth -- this would be a lunar eclipse. The Moon exhibits different phases as the relative geometry of the Sun, Earth and Moon change, appearing as a full moon when the Sun and Moon are on opposite sides of the Earth, and as a new moon (also named dark moon, as it is not visible at night) when they are on the same side.

Names of lunar phases

The phases of the Moon have been given the following names, in sequential order:

Phase Northern Hemisphere Southern Hemisphere Visibility
New moon Not visible, or traditionally, the first visible crescent of the Moon
Waxing Crescent moon . Right 1-49% visible Left 1-49% visible afternoon and post-dusk
First Quarter Moon Right 50% visible Left 50% visible most of the day and early night
Waxing gibbous moon Right 51-99% visible Left 51-99% visible afternoon and most of night
Full Moon Fully visible Fully visible sunset to sunrise (all night)
Waning gibbous Moon Left 51-99% visible Right 51-99% visible most of night and morning
Last Quarter Moon Left 50% visible Right 50% visible late night and most of the day
Waning Crescent Moon Left 1-49% visible Right 1-49% visible pre-dawn and morning
Darkened moon Not visible Not visible

The average calendrical month, which is 1/12 of a year, is about 30.4 days, while the Moon's phase (synodic) cycle repeats every 29.53 days. Therefore the timing of the Moon's phases shifts by an average of about one day for each successive month

Cumulus Webtag

Cumulus has three webtags relating the phase of the Moon.

Cumulus processes a webtag representing the current phase of the Moon. One of the following eight strings will be returned to <#moonphase>

  • New moon
  • Waxing Crescent
  • First Quarter
  • Waxing Gibbous
  • Full moon
  • Waning Gibbous
  • Last Quarter
  • Waning Crescent

These default values can be overridden by editing the Strings.ini file.

Cumulus also has two webtags relating to the percentage illumination of the Moon.

<#MoonPercent> returns the percentage of the visible surface illuminated, with positive values for a waxing Moon and negative values for a waning Moon.

<#MoonPercentAbs> returns the percentage of the visible surface illuminated, and is always positive.