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Updated March 14, 2011
Arietids (May 22 - July 8) &
the zeta Perseids (May 20 - July 5)
Although meteor observing is generally considered to be a night-time activity, there is no intrinsic reason why meteoroid streams should intersect only with the night side of Earth. Showers are generally associated with comets and, more rarely, Earth-crossing asteroids. In both cases the stream is equally likely to be on either the inbound part of its orbit where it approaches perihelion (closest approach to the Sun), or on the outbound, post-perihelion loop. In the latter cases the related meteors will appear to emanate from the general direction of the Sun, meaning the vast majority of them will occur during daylight hours. For some reason, about half (7 of 13) of the recognized daylight showers occur in May and June, when Earth's northern hemisphere is near its maximum tilt towards the Sun. In this report we will concentrate on the Arietids, the best daylight shower of the year, with occasional references to the zeta Perseids.
Arietids: May 22-July 2, maximum June 8, ZHR = 60
zeta Perseids: May 20-July 5, maximum June 13, ZHR = 40
The Zenith Hourly Rate (ZHR), is an assumed optical rate under ideal observing conditions. In reality such conditions are unattainable, but especially so for daylight showers. Nonetheless, the derived peak rate for the Arietids compares with the best of the consistent night-time showers (Perseids, Geminids, Quadrantids), and it is a longer lasting shower than any of these. Only the Leonids or Giacobinids under storm conditions would yield a significantly higher ZHR.
There has been considerable debate about the stream's inclination and the apparent daily motion of the radiant. Current theory suggests the confusion may have been caused by multiple filaments of the stream, not unlike recent conclusions about the Leonids.
The best match among known objects for a parent body of the Arietids is the Apollo asteroid 1566 Icarus. The Apollo asteroids are so-named because they have Earth-crossing orbits, as indeed one would expect for any object spawning an Earth-crossing meteoroid swarm. The origin of the zeta Perseids is less clear, but the shower may be a twin of November's Southern Taurids and therefore related in a convoluted way to the well-known Comet Encke.
The radiant point of both showers is somewhat east of the Sun, so it is possible to observe them visually. They can best be seen in the hours just before dawn, as grazing meteors which appear to rise from the northeastern horizon to cut a long majestic arc across the sky. Even experienced observers under good sky conditions record rates of only around one per hour, so consider yourself very fortunate to catch even one.
Daytime meteors are like nighttime birds: meant to be heard but not seen! Virtually all of our knowledge of daylight showers comes through radio observation. In a sense this is ‘observing with one eye closed’ in that radio and visual data cannot be compared. Indeed, despite its high ZHR, information about this shower is absent from most observing guides; as Kronk wryly states, ‘Other details concerning this shower have not been easy to obtain’. If anything, this underscores the importance of ongoing radio monitoring as the major source of information.
As is typical of most of the daytime showers, the first observations were recorded at England's Jodrell Bank in 1947. The Arietids have been detected in every major radar study since.
2003 Observation Plans
The June daylight showers will be subject to extensive radio observation by Sky Scan participants, as the shower holds considerable promise for a Grade 9 end-of-the-school-year project, with its emphasis on Unit V: Space Exploration. The multiple cross-curricular references to earlier units make a radio meteor project ideal as year-end review material. And its daytime nature means that observations can be made in classroom hours.
For more information
Further reference is made specifically to Gary W. Kronk's Comets and Meteor Showers page, at: