Moon & Rainbow

St�phane Pic caught this unusual combination of waxing moon and rainbow at Reunion Island in the Indian Ocean.

The �Moon Illusion� is well known. To our eye/brain, but not to cold measuring tools, it seems larger when near the horizon. It can also appear disproportionately large even when high in the sky. In fact its 0.5� diameter is tiny compared to the sky�s expanse. Here it is dwarfed by a rainbow. It does, however, provide a nice yardstick to confirm that the rainbow is indeed about 2� wide.

Image �St�phane Pic, shown with permission.

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The moon can only ever appear near a rainbow with this phase or the equivalent waning one.

Rainbows are ~42� in radius and centred directly opposite the sun at the antisolar point, ASP. The moon�s path takes it near to the ASP each month at full. That it almost never crosses the ASP is because the moon�s orbit is tilted ~5� to the ecliptic, the plane of Earth�s orbit around the Sun.

The low 5° inclination of the Moon�s orbit constrains where it crosses a primary rainbow.  Its path is always on a chord very roughly parallel to the ecliptic and within 5° of it.

The moon's rainbow crossing phase is always ~42° from full. Artists � beware of showing a crescent Moon near a rainbow!

The image was taken in the Southern Hemisphere and the moon has its northern pole downwards. The angle of the ecliptic to the horizon depends on location, season and time of day.