Eta Aquarids: Spring Meteor Showers Gifted to Earth From Comet Halley

Summary: The early May-peaking Eta Aquarids are annual spring meteor showers gifted to Earth from Comet Halley.

stray Eta Aquarid on May 10, 2013: Jason Jenkins (j-dub1980[THANK YOU FOR 100k+ Views], CC BY SA 2.0, via Flickr

The Eta Aquarids (pronunciation: ey-tuh, or ee-tuh + ak-wuh-rids, or ah-kwuh-rids) appear as the first of two annual meteor showers produced by Earth's passage through the trail of debris released by Comet Halley. Also known as Halley's Comet, the periodic comet with a predictable orbit of every 75 to 76 years is also responsible for autumn's Orionid meteor shower, which recurs annually from around October 15 to 29.

According to information compiled for over three decades by amateur astronomer organizations in Australia, England, Japan, New Zealand and the United States, about one-third of the Eta Aquarids produce trails of luminosity lasting for a second or longer.

The Eta Aquarids' annual shower commences around April 19 and continues until around May 28. Peak viewing occurs around May 5 to 6. The Eta Aquarids are considered to have a broad maximum, which means that optimal viewing immediately precedes and follows the predicted peak.

Although observable globally, the Halley-induced meteor shower is best viewed in the Southern Hemisphere, especially in Australia, where shower rates spike at 30 to 40 per hour. Viewers in the Northern Hemisphere, especially in Europe, Japan and the United States, are treated to displays rising from 10 per hour to a peak hourly rate of 20.

A meteor shower's name reflects the name of the constellation seemingly located nearest to the radiant, also known as apparent radiant, which is the point, from Earth's perspective, from which the shower appears to originate. The radiant for the Halley-associated April-to-May meteor shower appears to lie near Eta Aquarii (η Aqr, η Aquarii) in the constellation of Aquarius the Cup Bearer, or Water Carrier.

Eta Aquarii contributes to the shape of the constellation's water jar, along with Gamma Aquarii (γ Aqr, γ Aquarii), Pi Aquarii (π Aqr, π Aquarii), and the jar's central star, Zeta Aquarii (ζ Aquarii, ζ Aqr). As the 10th largest constellation, occupying a space of 980 square degrees, Aquarius is unmissable.

With its current placement as an equatorial constellation, Aquarius is one of 15 constellations through which the Earth's celestial equator passes. Because of Earth's axial tilt, the celestial equator currently traces an angle of 23.4 degrees with respect to the great, or Riemannian, circle that divides the Earth into two equal hemispheres. Although visible globally, equatorial constellations reach their upper culmination -- or highest point as viewed from Earth -- in the tropics. The equatorial-based region of the tropics is defined in the Northern Hemisphere by the Tropic of Cancer (23 degrees 26 minutes 16 seconds north 0 degrees 0 minutes 0 seconds west) and in the Southern Hemisphere by the Tropic of Capricorn (23 degrees 26 minutes 26 seconds south 0 degrees 0 minutes 0 seconds west).

Earth's two equators: inclination of Earth's celestial equator (green circle) with respect to great, or Riemannian, circle (black circle): Tfr000, CC BY SA 3.0, via Wikimedia Commons

During spring in the Northern Hemisphere, Aquarius appears in the eastern to southeastern skies. Optimal viewing of Aquarius in the Northern Hemisphere is in autumn.

Spring in the Northern Hemisphere equates to autumn in the Southern Hemisphere, which is the optimal season for viewing Aquarius in southern latitudes. During autumn in the Southern Hemisphere, Aquarius appears in the eastern to northeastern skies.

The best time for viewing the Eta Aquarid meteor showers during their peak occurrence is in the early morning hours before local sunrise.

The phase of the moon may facilitate or trouble viewing of the Eta Aquarid meteor showers. In 2015, the peak date of pre-dawn hours on Wednesday, May 6, coincides with a waning gibbous moon, which may distract from -- but not obscure -- the shower.

Optimal viewing strategies include locating sites where moonlight casts shadows, such as the side of a building, and focusing attention in darkened sky patches near, but not within, the radiant.

Even outside of prime viewing latitudes, the Eta Aquarids are worthy sights.

ca. 1824 depiction of Aquarius amidst celestial neighbors such as Capricorn and Pisces by British cartographer and engraver Sidney Hall (1788 – 1831): Library of Congress Prints and Photographs Division, Public Domain, via Wikimedia Commons

Acknowledgment

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

Image credits:

stray Eta Aquarid on May 10, 2013: Jason Jenkins (j-dub1980[THANK YOU FOR 100k+ Views]), CC BY SA 2.0, via Flickr @ https://www.flickr.com/photos/jdub1980/8726699547/

Earth's equators: Tfr000, CC BY SA 3.0, via Wikimedia Commons @ http://commons.wikimedia.org/wiki/File:Earths_orbit_and_ecliptic.PNG

depiction of Aquarius by British cartographer and engraver Sidney Hall (1788 – 1831) for Urania's Mirror; or, A View of the Heavens (32 astronomical star chart cards, first published in November 1824): Library of Congress Prints and Photographs Division, Public Domain, via Wikimedia Commons @ http://commons.wikimedia.org/wiki/File:Sidney_Hall_-_Urania%27s_Mirror_-_Aquarius,_Piscis_Australis_%26_Ballon_Aerostatique.jpg

For further information:

Kronk, Gary W. “Observing the Eta Aquarids.” Meteor Showers Online.
Available @ http://meteorshowersonline.com/eta_aquarids.html

McClure, Bruce. "Eta Aquarid Meteors Peak in Moonlight, Before Dawn May 6."Eta Aquarid meteors peak in moonlight, before dawn May 6." EarthSky > Tonight > May 5 - May 11. May 5, 2015.
Available @ http://earthsky.org/tonight/eta-aquarid-meteor-shower-best-before-dawn-may-6