First Quarter Moon: Third Phase of Eight Phase Moon Cycle Famous for Lunar X

Summary: The third phase of the eight phase lunar cycle, the first quarter, is famous for visibility of the Lunar X optical effect.

first quarter moon, visible high in southern sky in early evening: NASA Goddard Space Flight Center (NASA Goddard Photo and Video), CC BY 2.0, via Flickr

The first quarter moon appears as the third phase of the moon's eight phase monthly cycle. This phase marks the increasing illumination of the lunar disk as viewed by Earthlings. The first quarter presents a surface that is 50 percent illuminated.

The moon's eight phases are further classified into four primary and four intermediate phases. The first quarter stands as the second primary phase, with the new moon, which initiates each monthly lunar cycle, holding sway as the first primary phase. The first quarter is sandwiched between two intermediate phases. The waxing crescent is the first intermediate phase, and the waxing gibbous is the second intermediate phase.

The side of the moon that appears lit to Earthlings depends upon viewing above or below the equator. In the Northern Hemisphere, the right side of the first quarter is aglow. In the Southern Hemisphere, the left side of the first quarter shines.

The first quarter moon graces the noon to midnight hours. Rising around noon, the first quarter appears high in the sky at sunset. Its setting occurs around midnight.

In its first quarter phase, the moon is one-fourth of the way through its orbit around the Earth. The phase commences when the moon, with the Earth as the angle vertex, forms a 90 degree angle with the sun. The first quarter phases into the waxing gibbous moon when the moon and sun angle reaches 135 degrees.

The first quarter phase invites viewers to become familiar with surface features. For example, binocular or telescopic views of lunar craters take in three craters in the moon's south central region.

Ptolemaeus: With coordinates of 9.2 degrees south 1.8 degrees west, Ptolemaeus is located east of Mare Nubium in the moon's south central region. The low-angled sun that characterizes the first quarter and third quarter phases ideally highlights the ancient impact crater's features as a somewhat circular polygon with an irregular outer rim. Its dimensions measure 153 kilometers in diameter and 2.4 kilometers in depth. The crater's name honors Alexandria-based Greco-Egyptian astronomer and mathematician Claudius Ptolemy (ca. 90-168 CE).

Arzachel: With coordinates of 18.2 degrees south 1.9 degrees west, Arzachel is located east of Mare Nubium and south of Ptolemaeus in the highlands in the moon's south central region. The lunar impact crater's clear structure measures a diameter of 96 kilometers and a depth of 3.6 kilometers. Named by Italian Jesuit astronomer Giovanni Battista Riccioli (April 17, 1598-June 25, 1671), Arzachel honors Moorish astronomer Abū Isḥāq Ibrāhīm ibn Yaḥyā al-Naqqāsh al-Zarqālī (1029-1087).

Purbach: South of Arzachel lies Purbach crater, with coordinates of 25.5 degrees south 1.9 degrees west. The crater's heavily worn outer walls trace a diameter of 118 kilometers; its depth measures 3 kilometers. Purbach crater joins with Blanchinus and La Caille craters to create the Lunar X, also known as the Purbach cross, through a chiaroscuro optical effect most apparent at the beginning of the first quarter moon and also after the ending of the third quarter moon. Also named by Giovanni Riccioli, Purbach honors Austrian astronomer Georg von Peuerbach (May 30, 1423-April 8, 1461).

The first quarter phase offers rewarding views. A great gift for Earthlings seeking to expand their vision beyond the close parameters of modern life is the moon's unmissable visibility to the naked eye. No special paraphernalia is necessary for lunar sightings.

As with each lunar phase, the first quarter moon rewards viewers with its phase-specific light show. In addition to regaling unaided eyes, the first quarter moon invites closer, rewarding inspection by way of simple optical equipment, such as binoculars or small telescopes.

Lunar X (upper right): Jeff Barton (chipdatajeffb), CC BY 2.0, via Flickr

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

first quarter moon, visible high in southern sky in early evening: NASA Goddard Space Flight Center (NASA Goddard Photo and Video), CC BY 2.0, via Flickr @ https://www.flickr.com/photos/gsfc/15982765351/

Lunar X, imaged April 14, 2008, with a DMK 21F04 firewire camera + SV152 apochromat + Televue 4X PowerMate: Jeff Barton (chipdatajeffb), CC BY 2.0, via Flickr @ https://www.flickr.com/photos/chipdatajeffb/2413096589/

For further information:

Byrd, Deborah. "What is Lunar X?" EarthSky > FAQs > Space. Jan. 19, 2013.
Available @ http://earthsky.org/space/what-is-lunar-x

Chapman, David M.F. "The Lunar X Files: a fleeting vision near the crater Werner." Journal of the Royal Astronomical Society of Canada, Vol. 101, No. 2 (April 2007): 51 - 56.
Available via JRASC @ https://www.rasc.ca/sites/default/files/jrasc2007-04-lr.pdf
Available via Willingboro Astronomical Society @ http://wasociety.us/Lunar-X.pdf

Dickinson, David. "Stalking the Lunar X." Universe Today. March 18, 2013.
Available @ http://www.universetoday.com/100810/stalking-the-lunar-x/

Fuller, David. “Moon Maps.” Eyes on the Sky > Moon.
Available @ http://www.eyesonthesky.com/Moon.aspx

Grego, Peter. The Moon and How to Observe It. Astronomers’ Observing Guides Series. London: Springer-Verlag, 2005.

Marriner, Derdriu. "First Quarter Moon Arises Wednesday, February 25: Second Appearance in 2015." Earth and Space News. Wednesday, Feb. 25, 2015.
Available @ https://earth-and-space-news.blogspot.com/2015/02/first-quarter-moon-arises-on-wednesday.html

Marriner, Derdriu. "Lunar X Marks Spot of Optical Effect of Sunlit Craters at First Quarter Moon." Earth and Space News. Wednesday, March 25, 2015.
Available @ https://earth-and-space-news.blogspot.com/2015/03/lunar-x-marks-spot-of-optical-effect-of.html