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Posts Tagged ‘Solstice’

06/22/2015 – Ephemeris – The summer full moon and the winter Sun trade places

June 22, 2015 Comments off

Ephemeris for Monday, June 22nd.  Today the Sun will be up for 15 hours and 34 minutes, setting at 9:32.   The Moon, 2 days before first quarter, will set at 1:04 tomorrow morning.  Tomorrow the Sun will rise at 5:57.

Summer’s here, and it’s a few days before the latest sunset and latest end of twilight.  It might be instructive to check out the height of the moon over the next two weeks or so.  The moon is heading south in front of the Sun.  The Sun besides its apparent westward motion during the day caused by the Earth’s rotation also moves about twice its diameter each day eastward against the stars caused by the earth’s motion in its orbit of the Sun.  Around July 1st, the moon will be about where the Sun will be next winter solstice, 4 days before Christmas.  Actually it will be about 8 moon widths above where the Sun will be because its orbit is tilted a bit to the Earth’s.  But it will serve as an illustration of the seasonal difference.

Times are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

Summer full moon

The full moon nearest the summer solstice. The full moon appears near where the sun would appear low in the south at the winter solstice. The bottom red line is the ecliptic, the path of the Sun. Created using Stellarium.

Moon near the winter solstice

The full moon nearest the winter solstice. The full moon appears near where the sun would appear high in the south at the summer solstice. The top red line is the ecliptic, the path of the Sun. Created using Stellarium.

The Moon’s orbit has a slight tilt of a bit more than 5 degrees from the ecliptic, or plane of the Earth’s orbit of the sun.  The crossing point is called a node.  In the bottom image the node near the western horizon is called the descending node due to the fact that the Moon is heading south of the ecliptic.  When the Sun and Moon are near the same node the Moon will be new and we have a chance for a solar eclipse.  When at opposite nodes, a lunar eclipse.  The nodes slowly slide westward slowly one revolution in about 18.6 years, which causes eclipse seasons, about 6 months apart to occur a bit earlier each year.

Categories: Concepts, Seasons, The Moon Tags: , , ,

How come hours of daylight changes very slowly around the solstice, but very rapidly around the equinoxes?

January 10, 2015 4 comments

This question came in as a an off topic comment to my post yesterday 01/09/2015.  It deserves a good answer.  So here goes.

Day to day change in daylight hours occur when the Sun appears to move south or north.  For us in the northern hemisphere the daylight hours get shorter when the Sun appears to move south, and longer when the Sun appears to move north.  If we spread out the sky in a Mercator projection, like they do the earth or one of those satellite tracking maps, it would look like the image below.

Mercator map of the heavens

Mercator projection of the heavens from declinations +60 to -60 degrees declination, centered on the vernal equinox. The center horizontal white line is the celestial equator, and the yellow sinusoidal line is the ecliptic, the apparent path of the sun. Note the planets and Moon also stick close to that line. The date of the image is January 9, 2015. Venus and Mercury are on top of each other and unlabeled under the ‘a’ in Capricornus. Created using Cartes du Ceil (Sky Charts).  Click image to enlarge.

Note that the steepest part of the ecliptic occurs at the equinoxes, the vernal or March equinox in the center and the autumnal or September equinox at the left and right edges.  That’s where the sun’s motion north or south is the greatest, so the daily change in daylight hours is the greatest.  Near the solstices at 6 and 18 hours* the Sun isn’t changing its north-south motion very much, so the daylight hours aren’t changing much from day to day.  If you were watching the sky at local solar noon, you’d think that at the solstice the sun would stop its motion and stand still before heading back.  That’s what the word solstice means:  sun-standstill.  The variation is daylight hours also depends on your location.  At the equator, it doesn’t change at all.  Of course at the other extreme, at the poles, there’s 6 months of daylight and 6 months of night.

* The east-west direction in the heavens is like longitude on the Earth but it’s called right ascension and is measured in hours where 15 degrees equals one hours.  Astronomers use clocks to keep track of it.  Declination is the same as latitude on the Earth.  In astronomy longitude and latitude were already in use for ecliptic based coordinates.

So what causes the wavy path in the sky?  Lets check out the earth from the sun’s point of view, so to speak.

Earth's axial tilt.

Earth’s axial tilt. The horizontal line is the plane of the Earth’s orbit and what we see projected on the sky as the ecliptic. The tilt of the Earth’s axis to the plane of its orbit by 23 1/2 degrees, gives us the seasons and why the celestial equator and ecliptic cross at a 23 1/2 degree angle. Credit Dennis Nilsson.

Both the celestial equator and the ecliptic are great circles in the sky.  They intersect at an angle of 23 1/2 degrees at the equinox points.

Lets take a look at the difference in daylight hours at three times in the year, the equinox and the two solstices for Traverse City, MI whose latitude is just shy of 45° north.  The following three images were generated in stereographic projection, which exaggerates the distance of things near the horizon and diminishes the distance of things in the center, the zenith.  So actually the speed of the sun is unchanging across the sky.

Winter solstice

The sun’s daily path through the sky from horizon to horizon on the first day of winter, the winter solstice. Credit My LookingUp program.

Equinox

The sun’s daily path through the sky from horizon to horizon on an equinox the first day of spring or autumn. Credit My LookingUp program.

Note that at the equinox the sun rises due east and sets due west.

Summer Solstice

The sun’s daily path through the sky from horizon to horizon on the first day of summer, the summer solstice. Credit My LookingUp program.

One more diagram to illustrate the change in the sun’s north-south position in the sky.

Analemma

This figure 8 is called an analemma. One can find it on old globes in the Pacific Ocean. Created using my LookingUp program.

This is the Sun plotted for mean solar noon over one year at 7 day intervals.  One can see the rapid motion in the north-south position of the sun around the equinoxes versus the solstices.  The more rapid the north-south motion of the Sun the greater the change in day-to-day daylight hours.  The line with “East West” on it is the celestial equator.  Check out my December 2, 2014 post on why it’s a figure 8.

06/21/2013 – Ephemeris – Seasons and the height of the sun

June 21, 2013 Comments off

Ephemeris for Friday, June 21st.  Today the sun will be up for 15 hours and 33 minutes, setting at 9:31.   The moon, 2 days before full, will set at 5:11 tomorrow morning.  Tomorrow the sun will rise at 5:57.

Happy summer.  It began at 1:04 this morning.  The sun is at its highest at noon, well local solar noon that is, which is 1:44 p.m. in the Interlochen Traverse City area. At that time the sun will reach an altitude or angle above the southern horizon of nearly 69 degrees.  If you want to get an idea of the difference between that and the sun at the winter solstice, check out the moon tonight.  It is almost to the point in the sky where the sun was at the winter solstice.  Notice how low it is in the sky, and how few hours it is up.  The cause is the tilt of the earth’s axis of 23 and a half degrees.  It gives us a 47 degree span of altitudes of the sun over the year.  It is not the sun’s distance that causes seasons, as we’ll see next month.

Times are for the Traverse City/Interlochen area of Michigan.  They may be different for your location.

Addendum

Solstices

Comparing the sun’s path at the summer and winter solstices. This is a stereographic representation of the whole sky which distorts the sky and magnifies the size of the sun’s path near the horizon.

Categories: Ephemeris Program, Seasons Tags: