Power spectrum (IMAGE) University of Toronto Caption A power spectrum of the Earth's atmosphere. The x-axis is wavelength, e.g. 5 is a wavelength of one fifth of the circumference of the Earth for a wave traveling west to east, and -5 means the same, but for waves traveling east to west. The y-axis is frequency, in cycles per day, e.g. 2 means two cycles per day, or 12 hours. The thin horizontal brown lines show the Solar forcing at one, two, three, etc. cycles per day (24 hours, 12 hours, 8 hours period, and so forth). Credit Sakazaki & Hamilton Usage Restrictions Image for use in editorial coverage of the article ‘Why the day is 24 hours long; the history of Earth’s atmospheric thermal tide, composition, and mean temperature,’ published in the journal Science Advances. License Original content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.