Meteors from this stream may be observed during the interval of September 22 to October 23. Maximum occurs between October 6 and 15 from an average radiant of RA=84.8 deg, DEC=+51.9 deg. The radiant's daily motion is +1.2 deg in RA and +0.1 deg in DEC.
This meteor shower was discovered in 1979, while Jack D. Drummond, Robert K. Hill and Herbert A. Beebe (New Mexico State University) were determining the orbits of 13 meteors doubly photographed during 1976 and 1977 at the NASA-NMSU Meteor Observatory in southern New Mexico. Two of the 13 meteors had nearly identical orbits and had been photographed on October 13 and 18, 1977. It was concluded that the possible date of maximum might have occurred during October 15-16, 1977, from an average radiant of RA=95.7 deg, DEC=+52.5 deg.
The first observations of this shower occurred during October 1980, as Norman W. McLeod, III (Florida) and Drummond (New Mexico) carried out independent visual surveys. McLeod's observations began early as he noted 1 meteor in 4.6 hours on October 6, 1 in 0.8 hours on the 10th and 0 in 2.7 hours on the 12th. Both men observed on the 13th, with McLeod noting 2 in 1 hour and Drummond detecting 3 in 2 hours. Drummond's observations of the 14th and 15th revealed 2 in 2 hours and 5 in 2 hours, respectively---the latter being the highest determined hourly rate of 2.5 per hour. Observations continued from the 16th to the 22nd, with the Delta Aurigids being either nonexistent or possessing hourly rates of 1 or less.
During 1981, Drummond isolated 5 probable members of this stream that had been photographed by American and Russian surveys conducted between 1950 and 1962. Combining these 5 with the 2 meteors from the 1977 NASA-NMSU survey, Drummond concluded that the shower's duration might be from September 29 to October 18. The mean date of activity was October 8, with the radiant then being RA=87.8 deg, DEC=+50.2 deg. The shower's daily motion was given as +1.0 deg in RA and +0.1 deg in DEC. Lastly, the stream's mean orbit, based on the 7 photographic meteors, is a retrograde orbit with a period of 115 years.
From both the photographic studies and the visual survey of 1980, Drummond pointed out that only the second half of the indicated shower duration had been covered and that the determination of the shower's true maximum "must await further observation from the first half."
The Author has recently searched through the 39,145 radio meteor orbits obtained during the two sessions of the Radio Meteor Survey and isolated 24 probable Delta Aurigids. The orbital details will be further discussed in the "Orbit" section below, but several interesting features can be discussed here.
First, while comparing the radio meteor orbits with the photographic orbits, it becomes apparent that the radio meteors seem to reach peak concentration about one week before the photographic meteors. The radio meteors also possess a much smaller semimajor axis of 2.3 AU, compared to the photographic meteors' semimajor axis of 18.7 AU. At first glance, it might seem that there is a significant indication of mass distribution within this stream, but when all of the data was combined and reexamined it appears that the following very distinct filaments may be present in this stream:
- Filament "A" reaches maximum on September 30 (solar longitude=186.5 deg), from an average radiant of RA=87.8 deg, DEC=+54.1 deg. It possesses the largest perihelion distance of the four possible filaments and was composed of 11 meteors---one being photographic.
- Filament "B" reaches maximum on October 7 (solar longitude=193.2 deg), from an average radiant of RA=87.5 deg, DEC=+50.2 deg. It possesses the largest semimajor axis of the four filaments (4.5 AU) and is the probable main core of this stream. It was based on 13 meteors, with 6 being photographic.
- Filament "C" is the weakest filament, but seems to reach maximum on October 13, from an average radiant of RA=91.1 deg, DEC=+47.8 deg. It possesses the highest inclination (133 deg) and the smallest perihelion distance (0.75 AU) of the four filaments. It was based on 4 meteors, with 2 being photographic.
- Filament "D" reaches maximum on October 2 (solar longitude=188.2 deg), from an average radiant of RA=74.3 deg, DEC=+55.0 deg. It possesses the smallest inclination (116 deg) and the smallest semimajor axis (2.2 AU) of the four filaments. It was based on 5 radio meteor orbits.
Searches for previous sightings of the Delta Aurigid shower have not proved extremely successful, though one very promising observation occurred during October 14-17, 1876, when William F. Denning plotted 6 meteors from an average radiant of RA=90 deg, DEC=+58 deg. The meteors were described as rapid, with streaks. This radiant is the closest 19th century match for the Delta Aurigids, but is only borderline according to the D-criterion.
More positive observations of the Delta Aurigid stream are found in Cuno Hoffmeister's Meteorstrome. Three excellent radiants are given for October 1, 1910 (RA=86 deg, DEC=+57 deg), October 9, 1920 (RA=84 deg, DEC=+48 deg), and October 7, 1935 (RA=83 deg, DEC=+48 deg). The first radiant seems a good example of a filament "A" shower, while the latter two radiants probably belong to filament "B". Seven additional radiants also appear, but are strictly borderline, according to the D-criterion. These seven radiants might belong to filaments "A", "B" and "C". Filament "D" continues to be elusive in the records of visual observations and may be composed mostly of small, telescopic particles---if it exists at all.
Interestingly, despite these indications of visual observations, there is no appearance of this shower in the records of the American Meteor Society, or most other catalogs of visual radiants.