Ephemeris for Monday, March 13th. The Sun will rise at 7:58. It’ll be up for 11 hours and 48 minutes, setting at 7:46. The Moon, 1 day past full, will rise at 9:03 this evening.
We are now plunged back into dark mornings like we were two month’s ago thanks to the start of Daylight Saving Time. However we are only a week from the vernal equinox, the first day of spring here in the northern hemisphere. However some of my blog followers down under will experience the start of autumn on that day. For us in the next three months the sunrise time will back down 2 hours, and will rise around 6 a.m. Our sunset times will advance a bit less than that, an hour and 45 minutes. The lopsidedness is a consequence of both the Earth’s axial tilt and its slightly elliptical orbit. We are moving somewhat away from the Sun now and are slowing down a bit. It’s all kind of hard to explain, but makes perfect sense… eventually.
Times are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Well, I’m going to try to explain it now.
If one visits most observatories, there will be a clock, usually in the dome that doesn’t seem to read the correct time. We have one in Northwestern Michigan College’s Rogers Observatory’s dome. It only agrees with the time on your watch for an instant on October 16th. It’s called a sidereal clock and it measures Earth’s rotation with respect to the stars, and it gains approximately 4 minutes a day compared to our normal clocks which are geared to the Sun.
The Earth and all the planets orbit the Sun in a counterclockwise direction when seen from the north. Also the Earth and most of the planets spin also in a counterclockwise direction. The exceptions are Venus and Uranus. Most large satellites like our Moon also orbit their planets in a counterclockwise direction. From the surface of the Earth these bodies appear to generally move eastward, while the celestial sphere mirror reflects our eastward rotation by appearing to move east to west.
The Sun appears to move about one degree a day eastward. (360° / 365 days = 1° approximately). The Earth rotates through 15° an hour (360°/24 hours = 15° and 60 minutes in an hour / 15 = 4 minutes to rotate a degree). So on average and approximately after the Earth rotates back to the same point with regard to the stars, it has to rotate and extra degree to catch up to the Sun, which takes 4 more minutes, which is why the solar day is 4 minutes longer than a sidereal day. (Well, 3 minutes 55.9084 seconds if you want to get picky about it!)
Of course it’s not that simple. It never is that simple. This would all work out if the Earth orbited the Sun in a uniform circle and the Earth had no axial tilt. The Earth’s tilt is also called obliquity. The Sun would appear to move uniformly over the Earth’s equator. That Sun, called the mean Sun is what we base our solar time on, not the real Sun. However the Earth’s orbit is elliptical, with the Earth moving fastest at its perihelion or closest point to the Sun, around January 3rd, and slowest at aphelion or farthest point around July 4th. Also the Earth’s axial tilt is 23 ½ °, and is only on the equator two days a year March 20th and September 23rd.
Ever see this funny figure 8 in the Pacific Ocean on old globes?
Both eccentricity and obliquity work together to produce the analemma as seen in the diagram below. This figure 8 can actually be photographed in the sky by exposing the same frame of film at regular intervals, or stacking images of the same area of the sky over a year at the same time of day. There are plenty of examples using an Internet search engine to search for analemma images. It is one way to illustrate the equation of time, which is the correction one must make to a sundial reading to get to the correct local mean solar time. To that one must add or subtract one’s offset from the time zone’s time meridian. See Friday’s post.
The above diagram was taken from Ethan Siegel’s Starts with a Bang blog: http://scienceblogs.com/startswithabang/2010/12/17/celebrate-this-winter-solstice/ which also explains it.
The equation of time can be found in tabular form for easy sundial correction, or in a linear graphical form as seen below.
Actually the biggest effect on the equation of time, is especially near the solstices is the Earth’s obliquity (axial tilt). Below we see how the Sun’s declination affects how fast it appears time wise.
Declination of celestial objects is the same as latitude on the Earth. A star whose declination is the same value as one’s latitude will cross at the zenith once a day. In illustration 4 note that the near the solstices the time lines (meridians) are closer together, so the Sun will pass them faster than when near the equinoxes where the time lines are farther apart and the Sun is moving somewhat diagonally, taking longer to cross the time lines. In time only east-west motion counts.
Looking at Illustration 2, I’ve added arrows at the top (northern) and bottom (southern) extremities of each analemma source to give some idea of the Sun’s apparent speed at the solstices. At the bottom, near the December solstice the eccentricity speed of the Sun adds to the obliquity speed increasing the effects at that part of the analemma lobe, making it bigger. At the June solstice end of things eccentricity speed is in the opposite direction, slowing the Sun down.
So what? This affects the dates of the earliest and latest sunrises and sunsets. Here are those dates and time values for us here in northern Michigan (specifically the Interlochen/Traverse City area):
All this may make little difference to our modern lives, governed by the atomic clocks in Paris and Fort Collins, Colorado, divorced as they are from the Earth’s actual rotation and the Sun except for the inclusion of the occasional leap second, like we had last December 31st. To folks like me who are amateur astronomers and have (or had in my case) a day job, it would’ve been nice to have, on the summer solstice, astronomical evening twilight end before midnight.
I hope this helped rather than confused you. What do you think? drop me a comment.