The activity of this stream persists from February 5 to March 19. The shower reaches maximum on February 22, from an average radiant of RA=156°, DECL=+18°. The ZHR is 3, while the average magnitude of the meteors is near 2.86. A possible telescopic southern branch may have a duration extending from January 13 to February 24, with a maximum on February 3 and an average radiant of RA=135°, DECL=+8°.
This shower appears to have been first noted during the 20th century, since it is completely absent in the 19th century records of W. F. Denning, A. S. Herschel, R. P. Greg and E. Heis.
The first observation of a shower from this stream appears in the records of Denning for February 19-March 1, 1911. Seven meteors were plotted from an average radiant of RA=155°, DECL=+14°. The meteors were described as slow, with trains. A meteor six times the brightness of Venus was observed by 16 observers on February 28.8, 1910. The radiant was determined as RA=155°, DECL=+16°.
Additional visual observations belonging to this meteor shower were obtained in 1924 and 1930. In the former year, J. P. M. Prentice plotted several meteors from RA=155°, DECL=+13°, during February 25-28. During 1930, two independent observations were made from opposite sides of the Atlantic. On February 19, C. Hoffmeister (Germany) detected a radiant at RA=149°, DECL=+18°, while observations by B. S. Whitney during February 20-21, revealed a radiant at RA=154°, DECL=+21°.
The strongest arguments for this stream's existence have been its detection using both photography and radio-echo methods during the 1950's and 1960's. The former technique was best carried out during the Harvard Meteor Project of 1952-1954. The data was analyzed using a computer during 1971, by B.-A. Lindblad (Lund Observatory, Sweden), with 24 photographic members of the Delta Leonids being detected. He gave the period of visibility as February 5 to March 19, and determined the average radiant position as RA=159°, DECL=+19°. Although Lindblad suggested a relationship with either radiant 120 or 129, which appeared in Denning's 1899 "General Catalogue of the Radiant Points of Meteoric Showers and of Fireballs and Shooting Stars observed at more than one Station" as two long-duration stationary radiants, the Author has found no individual observations from these two showers which occur at a time and position that corresponds to the Delta Leonid radiant. It should also be pointed out that the Author believes several members of Lindblad's Delta Leonids are probably members of the Beta Leonids of March.
An ambitious radio-echo survey was carried out at Havana, Illinois, during 1961-1965. Called the Radio Meteor Project, it was headed by Z. Sekanina, who published the results during 1973. Sekanina isolated the Delta Leonid stream and said they existed during the period of February 9 to March 12. The date of nodal passage was given as February 19.9 (λ=330.7°), while the average radiant was RA=154.3°, DECL=+18.3°. Combining the 8 radio meteors used to establish this orbit, as well as the photographic meteors, the Author determined the radiant's daily motion as +0.93° in α and -0.38° in δ.
It should be noted that the 1968-1969 session of the Radio Meteor Project detected a stream similar to the Delta Leonids, but the orbit's argument of perihelion and ascending node were about 180 deg off. In addition, the duration was given as January 13 to February 24, the date of the nodal passage was February 2.3, and the radiant position was RA=135.2°, DECL=+7.5°. These figures all exhibit notable differences from that already established above for the Delta Leonids and, when compared to that stream's radiant ephemeris, this stream lies about 15 deg to the south. No trace of this possible southern branch of the Delta Leonids appears in visual and photographic records, which implies that it may be a possible telescopic shower.
Observations by members of the Western Australia Meteor Section (WAMS) have supplied some of the most valuable information on the Delta Leonids in recent years. Jeff Wood, director of the WAMS, has analyzed several years of observations and has concluded that maximum occurs on February 26, from RA=158 deg, DECL=+17 deg. He gives the duration as February 1 to March 13. In 1979, Delta Leonids were observed during February 22 to March 3. A maximum ZHR of 3.08+/-1.78 came on February 25. The radiant position was then given as RA=159 deg, DECL=+19 deg. The average magnitude of seven observed meteors was 2.86. During 1980, activity was observed during February 15 to March 9. A maximum ZHR of 2.59+/-0.57 came on February 22, at which time the radiant was at RA=159°, DECL=+18°.
Compared to other currently active meteor streams, the Delta Leonids appear to possess a fairly short history. During 1985, K. Fox (Queen Mary College, England) investigated the past and future orbits of 53 meteor streams. He found that the Delta Leonid orbit does not come in contact with Earth's orbit 1000 years in the past or future. Thus, this stream is only a temporary feature as far as Earth is concerned.