Pasadena Sunrise, Local Noon, Sunset Times and Times of Maximum UV Exposure

The following plots show the sunrise, local noon, sunset times and times of maximum UV exposure for Pasadena, CA. Daylight time is used when applicable, which accounts for the 1 hour jumps in the times near the beginning of April and the end of October. The sunrise and sunset data were obtained from the US Naval Observatory specifically for the location of Pasadena (W118o 08', N34o 09') from USNO Sunrise/Sunset Computation. The ultraviolet times come from calculations for the latitude of Pasadena using a formula from a Sky & Telescope article for which I embarrassingly do not have the reference handy.

The times for anyplace in the San Gabriel Mountains will not vary by more than 2-3 minutes from those values. (For example, compare with the sunrise/sunset times for Fallbrook, CA, which is ~52 miles south and ~52 miles east of Pasadena, a greater distance than anyplace in the SGM from Pasadena.)

Local noon is the time on your watch when the sun is actually at its highest position in the sky and appears due south. This time is different from 12:00 for two reasons:

  1. Pasadena is about 7 minutes of time east of the reference longitude (W120o) for the Pacific Time Zone. (The conversion is 4 minutes of time for every degree of longitude difference.)

  2. The variation about that 7 minute mean difference is due to the Equation of Time. That title sounds fancy, but the Equation of Time simply tells you the change in the time of local noon due to the ellipticity of the Earth's orbit around the sun and due to the tilt of the Earth's equator from the ecliptic. If the Earth's orbit were perfectly circular, and the Earth's equator were in Earth's orbital plane, the equation of time would be exactly zero. Since the orbit isn't circular, the equation of time changes more rapidly near perihelion (closest point to sun) which occurs in early January, and slowest in July. This yearly periodic change is modulated by a twice-yearly change due to the tilt of the Earth's axis.

    The Equation of Time also explains why the earliest sunset occurs in early December and the latest sunrise in early January, a month apart.

    See Bob Urschel's wonderful explanation of the Equation of Time and the Analemma.

Note the amazing thing that we have done with daylight savings time: we have made the range of sunset 3.25 hours, while keeping the range of sunrise to less than 1.5 hours.

The final plot below shows the times at which the UV reaches 0.4 times the maximum level it ever reaches (at noon on June 21). The value 0.4 was picked because that corresponds closely with the conventional wisdom to "stay out of the sun between 9 a.m. and 3 p.m.". Note that this advice refers to local noon, not noon daylight savings time, and is only really valid for May 21 to July 21. During other months, that same UV level is reached only much closer in time to local noon.

(Click on graph for bigger and better image.)

(Click on graph for bigger and better image.)

(Click on graph for bigger and better image.)

(Click on graph for bigger and better image.)

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Copyright © 1997-2000 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Last update: 15 February 2000