Archive

Posts Tagged ‘Daylight hours’

04/06/2018 – Ephemeris – Marking the passage of 13 hours of daylight

April 6, 2018 Comments off

Ephemeris for Friday, April 6th. The Sun will rise at 7:14. It’ll be up for 13 hours and 2 minutes, setting at 8:16. The Moon, 2 days before last quarter, will rise at 2:33 tomorrow morning.

Tomorrow morning early, the crescent Moon will pass Saturn and Mars. These planets will be below the Moon in the dark early morning hours. The dark night hours will be increasingly more inaccessible as summer approaches. Today we’ve broached 13 hours of daylight. By the summer solstice on June 21st the Sun will be out just a bit over 15 and a half hours. Meaning that the Sun will be down for only eight and a half hours, with only three and a half hours of really dark sky, Moon permitting, between the end of evening astronomical twilight and the beginning of morning astronomical twilight. Twilight is really long around the summer solstice because the Sun sets at a shallow angle.

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

Addendum

Definitions

End or start of Civil Twilight:  Sun is 6° below the horizon

Brighter planets become visible

End or start of Nautical Twilight:  Sun is 12° below the horizon

Brighter deep sky objects can be found for public star parties

End or start of astronomical twilight:  Sun is 18° below the horizon

On moonless nights, the twilight glow is gone and the sky is dark

Advertisements

07/02/2015 -Ephemeris – A belated preview of July’s skies

July 2, 2015 1 comment

Ephemeris for Thursday, July 2nd.  Today the Sun will be up for 15 hours and 30 minutes, setting at 9:31.   The Moon, 1 day past full, will rise at 9:48 this evening and tomorrow the Sun will rise at 6:02.

Lets preview July’s skies a day late.  Sorry, it’s been a busy week.. The sun, having reached its northern solstice, is beginning to slide southward again, at first imperceptibly, then with greater speed.  The daylight hours will decrease from 15 hours and 30 minutes Today to 14 hours 44 minutes at month’s end.  The daylight hours will be slightly shorter south of Interlochen, and slightly longer to the north.  The altitude of the sun at local noon, when the sun is due south will decrease from 68 degrees Now to 63 degrees at month’s end.  The sun will be a degree lower in the Straits area.  Despite the warmth, the earth will reach its greatest distance from the sun on Monday the 6th.  The range of the earth’s distance from the sun is 3 million out of 93 million miles.

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

Addendum

July Star Chart

Star Chart for July 2015. Created using my LookingUp program.  Click on image to enlarge.

The Moon is not plotted.

The planets and stars are plotted for the 15th at 11 p.m. EDT.  That is chart time.  Note, Traverse City is located 1 hour 45 minutes behind our time meridian.  To duplicate the star positions on a planisphere you may have to set it to 1 hour 45 minutes earlier than the current time.

Evening Astronomical twilight ends at midnight. EDT on July 1st, decreasing to 11:14 p.m. EDT on the 31st.

Morning astronomical twilight starts at 3:32 a.m. EDT on July 1st, and increasing to 4:42 a.m. EDT on the 31st.

Add a half hour to the chart time every week before the 15th and subtract and hour for every week after the 15th.

For a list of constellation names to go with the abbreviations click here.

The green pointer from the Big Dipper is:

  • Pointer stars at the front of the bowl of the Big Dipper point to Polaris the North Star.
  • Drill a hole in the bowl of the Big Dipper and the water will drip on the back of Leo the Lion.
  • Follow the arc of the Big Dipper’s handle to Arcturus
    • Continue with a spike to Spica
  • The Summer Triangle is shown in red

Calendar of Planetary Events

Credit:  Sky Events Calendar by Fred Espenak and Sumit Dutta (NASA’s GSFC)

To generate your own calendar go to http://eclipse.gsfc.nasa.gov/SKYCAL/SKYCAL.html

Times are Eastern Daylight Time on a 24 hour clock.  Some additions made to aid clarity.

Conjunctions like the Moon-Jupiter: 4.5° N means Jupiter will appear 4.5° north of the Moon.

 Date       Local   Event
             Time
Jul  01     We    02:48    Moon South Dec.: 18.4° S
     01     We        Venus: 42.4° E
     01     We    22:20    Full Moon
     05     Su    14:54    Moon Perigee: 367100 km
     06     Mo    08:59    Aphelion: 1.0167 AU
     07     Tu    20:07    Moon Descending Node
     08     We    16:24    Last Quarter
     12     Su    13:55    Moon-Aldebaran: 0.9° S
     14     Tu    00:24    Moon North Dec.: 18.4° N
     14     Tu    17:35    Venus-Regulus: 2.3° S
     15     We    21:24    New Moon
     18     Sa    13:34    Moon-Jupiter: 4.5° N
     18     Sa    21:06    Moon-Venus: 0.5° N
     21     Tu    07:02    Moon Apogee: 404800 km
     21     Tu    15:32    Moon Ascending Node
     23     Th    15:18    Mercury Superior Conjunction with the Sun
     24     Fr    00:04    First Quarter
     26     Su    04:43    Moon-Saturn: 2.4° S
     28     Tu    10:23    Delta Aquarid Meteor Shower: ZHR* = 20
     28     Tu    13:34    Moon South Dec.: 18.3° S
     31     Fr    06:43    Full Moon
Aug  01     Sa        Venus: 21.5° E

*ZHR – Zenithal Hourly Rate:  Approximate number of meteors per hour when the shower radiant is at the zenith.  For more information on this and other meteor showers in 2015 see the International Meteor Organization website calendar section: http://www.imo.net/calendar.

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.