On the double-peak spectrum of the Chandler wobble

It is well known that two close spectral peaks with comparable amplitudes exist in the Chandler wobble (CW) band in the observed polar motion spectrum during the first half of the last century. However, it is not generally accepted that the CW is an oscillation with two closely lying frequencies, and some authors attribute the appearance of these two peaks to some unusual phase variation (180 degrees shift) of the CW during the 1920s-1930s. The phase variation of the CW is explored using the longest Earth orientation parameter (EOP) time series currently available-the International Earth Rotation and Reference System Service (IERS) EOP-C01 from 1846 to 2003. The observation time series containing mainly the CW obtained by removing the annual wobble (AW) from the original EOP time series is compared with a synthetic double-frequency CW time series constructed using the frequencies, amplitudes and initial phases of the two peaks precisely determined using spectral analysis. The result shows reasonable agreement between their peak times, a feature not shown before. This feature is in agreement with the hypothesis of a double-frequency CW. While the result is inconclusive with regard to whether the CW really has two frequencies, it serves as a reminder of the renewed interest in the topic.