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Posts Tagged ‘Seasons’

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.

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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.

07/10/2014 – Ephemeris – Why is the bright Moon so low in summer and so high in winter?

July 10, 2014 1 comment

Ephemeris for Thursday, July 10th.  Today the sun will be up for 15 hours and 20 minutes, setting at 9:28.   The moon, 2 days before full, will set at 5:22 tomorrow morning.  Tomorrow the sun will rise at 6:08.

If you watch the moon for the next few nights, you won’t have to strain your neck because the moon at its highest will be less than 30 degrees above the southern horizon for us in northern Michigan.  That’s because the moon closely follows the path of the sun in the sky, called the ecliptic, with a deviation of only 5 degrees maximum.  Tonight it’s a couple of degrees north of the ecliptic.  Tonight it’s located about where the sun was back last November or will be next November.  In winter you’d swear that the full moon at its greatest height was practically overhead.  It’s another effect of the Earth’s axial tilt of 23 ½ degrees.  Our moon is odd in it doesn’t orbit the Earth’s equator like most large moons do for their planets.

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

Addendum

July Full Moon

The full Moon on July, 12, 2014. Created using Stellarium.

December Full Moon

The full Moon on December 6, 2014. Created using Stellarium.

 

06/14/2013 – Ephemeris – Earliest sun rise of the year

June 14, 2013 Comments off

Ephemeris for Flag Day, Friday, June 14th.  Today the sun will be up for 15 hours and 32 minutes, setting at 9:29.   The moon, 2 days before first quarter, will set at 12:51 tomorrow morning.  Tomorrow the sun will rise at 5:56.

The sundial makes a poor clock.  It will give the approximate time, but even if you could make the lines on its face accurate for one day, it could be off a week later.  The reason I mention this is because tomorrow morning is the earliest sunrise.  We’ll be still 6 days from the longest day, when the daylight hours are at their maximum.  The latest sunset will be on the 26th, 5 days after the summer solstice.  The disparity is even greater in the winter.  Sundials can be corrected by a table of how much the sun is fast or slow, called the equation of time.  It is caused by both earth’s slightly eccentric orbit of the sun, and the tilt of the earth’s axis.  It seems when you study things closely they are not as simple as they appear.

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

 

 

Categories: Ephemeris Program, Seasons Tags:

01/01/2013 – Ephemeris – We start the year with the sun closest to us.

January 1, 2013 Comments off

Ephemeris for New Years Day, Tuesday, January 1st 2013.  The sun will rise at 8:19.  It’ll be up for 8 hours and 53 minutes, setting at 5:13.   The moon, 3 days before last quarter, will rise at 9:55 this evening.

Happy New Year and happy perihelion Day.  What’s a perihelion?  Well. It’s not two helions, whatever they are.  Perihelion is the point in an orbit of the sun that is nearest to the sun.  This evening the earth will reach that point in its travels around the sun.  Since its winter time, the sun’s distance apparently doesn’t cause the seasons.  Actually the sun now as about a million and a half miles closer than mean, which isn’t much out of 93 million miles.  But, although we don’t notice it in northern Michigan, it makes winter the shortest season at 89 days.  The real cause of the seasons is the tilt of the earth’s axis.  Now the sun appears highest over the southern hemisphere.  The sun’s up less than 9 hours here and hangs low over the southern horizon at noon.

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

 

03/18/11 – Ephemeris – Spring is almost here

March 18, 2011 Comments off

Ephemeris for Friday, March 18th.  The sun will rise at 7:49.  It’ll be up for 12 hours and 2 minutes, setting at 7:52.   The moon, 1 day before full, will set at 7:16 tomorrow morning.

We are now two days and some hours from the beginning of spring.  Spring will arrive at 7:21 p.m. Sunday evening.  At that instant the sun will appear to cross the celestial equator, the projection of the earth’s equator on the sky, heading northward.  This will give us six months of over 12 hours daylight, culminating on June 21st with over 15 and a half hours of daylight for our listening area.  This is the spring or vernal equinox, or to be hemisphericaly correct, the March equinox, because those folks south of the equator will begin autumn.  Earth’s seasons are due to the tilt of its axis by 23 and a half degrees.  The earth’s axis is nearly fixed in space, but changes its orientation with respect to the sun during our yearly orbit.

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