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The Ashland Astronomy Studio Moon Calendar Poster shows in monthly columns how the Moon will appear to North American observers for every date of the year. The most obvious daily change in the Moon’s appearance is its phase. Moon phases complete a lunation cycle every 29½ days. The lunation cycle begins with a New Moon—when the Moon’s orbit around the Earth brings it closest to the Sun. Afterwards, the Moon will be sighted as a waxing crescent, a first quarter moon (exactly half-lit from the west), a waxing gibbous, and then as a Full Moon at the halfway point of the lunation cycle. After the Full Moon, it will be seen as waning gibbous, a last quarter moon (exactly half-lit from the east), and finally as a waning crescent until the next New Moon. Scan the monthly columns on the Moon Calendar itself from top to bottom to follow the entire sequence.

Another daily change that is easy to follow is how far north or south the Moon rises and sets: its declination. Like the Sun, the Moon rises in the east and sets in the west, but not exactly due east and due west except on special dates. As the Earth orbits the Sun yearly and the Moon orbits the Earth monthly, the sky positions of booth these heavenly orbs will veer noticeably north and south. This is a consequence of the changing relationship between the Sun’s or Moon’s direction in space with respect to the fixed direction of the Earth’s axial tilt. For North American observers, whenever the Sun or Moon rises at its northernmost declination, this is when it will climb highest in the sky. At its southernmost declination, the Sun or Moon will hang low in the southern sky. The Sun’s maximum declinations coincide with the annual summer and winter solstice (denoted on the Moon Calendar byand). The Moon’s maximum declinations are visited monthly (denoted by
N  and S ).

Each image on the Moon Calendar is printed in a “north is up” orientation. The actual Moon will match this orientation at your observing location when it crosses the meridian (an imaginary line in the sky running from north to south, dividing the sky into eastern and western halves). You will notice that the Moon’s axial orientation oscillates, depending on what constellation that the Moon appears in (indicated on the lower right). This is another effect due to the Moon’s changing position in space with respect to the Earth’s axial tilt. Note to advanced astronomers: the much more subtle pitching and yawing oscillations of the Moon, due to libration, are also precisely modeled in every image.

The apparent size of the Moon also changes in regular cycles. For observers on Earth, the Moon may look bigger when it is very close to the horizon, but this effect is an optical illusion, created by the mind from the landscape in the foreground which gives the Moon’s great distance an Earthly context. When the Moon is high in the sky, this context is lost and the Moon’s disk seems diminished. Nonetheless, the apparent size of the Moon (its angular diameter) does actually change. This is because the Moon’s distance from the Earth varies. On average, the Moon is 238,855 miles away from Earth, but it may come as close as 221,457 miles (at perigee) or drift as far out as 252,712 miles (at apogee). This approximately 30,000-mile variation occurs monthly. Whenever a Full Moon coincides with perigee, it is popularly known as a “supermoon.” A supermoon floods the Earth with the brightest moonlight of the year. The angular diameter of the Moon at perigee is 14% larger than it is at apogee—approximately the same difference in size (to coin an analogy) between a U.S. quarter and a nickel. The images printed on the Moon Calendar are precisely calibrated to match the angular diameter of the Moon itself when the calendar is viewed from a distance of exactly eight feet. Can you discern the difference in printed size between the biggest and smallest Full Moons? With enough viewing practice, you will be able to discern changes in the apparent size of the Moon in the sky.

Among the most spectacular and widely anticipated heavenly phenomena are eclipses. A solar eclipse occurs when the Sun, Moon, and Earth happen to line up directly in space in that order, so that the Moon casts a shadow on the Earth; a lunar eclipse occurs when the order of the Earth and Moon is reversed, so that the Earth casts a shadow on the Moon. The rarity of eclipses is due to another nuance in the Moon’s orbit: its orbital inclination. Because the Earth’s orbit around the Sun defines a plane (the ecliptic) and the Moon’s orbit around the Earth defines a separate, tilted plane, the Moon’s orbit intersects the ecliptic at two special places, called nodes (denoted by the symbolsand). This line of nodes aligns with the Sun at approximately six month intervals. Only when a New or Full Moon lies on a node can an eclipse occur—otherwise the Moon’s shadow will entirely miss the Earth or vice versa. If a shadow partially hits the Earth or Moon, a partial eclipse results. The Moon Calendar depicts how eclipses will appear to North American viewers. Eclipses which can only be observed outside North America are not depicted.

The Moon’s path near the ecliptic carries it through the twelve constellations of the zodiac and a few adjacent constellations. Here is a list of abbreviations for constellations indicated by the Moon Calendar (bold type indicates zodiacal constellations).

Aqr – Aquarius Leo – Leo Sco – Scorpius
Ari – Aries Lib – Libra Sex – Sextans
Cap – Capricornus Oph – Ophiuchus Sgr – Sagittarius
Cnc – Cancer Ori – Orion Tau – Taurus
Gem – Gemini Psc – Pisces Vir – Virgo


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Ashland Astronomy Studioo
2305 Ashland St. Ste. C PMB 312; Ashland, Oregon 97520

contact astrostudio.org

Tel: (541) 201-8854