June 2013 Solstice and Full Moon

(NOTE: The images below are screenshots from a program called Stellarium, available for free at www.stellarium.org. Click the image for a larger view)

This year (2013), the summer solstice and the June full moon occur within days of each other.  The noon solstice sun (shown in the top image crossing the meridian of my home in western Connecticut with the effects of the atmosphere removed) will have reached its highest point in the sky in 2013.  Note its position on the point of the ecliptic (drawn in red here) that is as far north of the celestial equator (drawn in blue) as can be.  The June 21 sun is in the middle of the ‘winter hexagon’, just above Orion and the star Betelgeuse, and happens to be crossing the plane of the Milky Way galaxy as well.  Venus and Mercury are to the left (east) of the sun, and Jupiter and Mars are just to its right, though none of that will be visible through the sun’s glare.  It’s interesting to note too that the autumnal equinox is just rising due east, and the vernal equinox just setting due west at noon on the summer solstice.

The June full moon, 2 days later, is opposite the sun, occupying the position of the winter solstice at the southernmost point of the ecliptic.  – it will be low in the southern sky all night, following the same path the December sun followed six months ago. And as that full moon crosses your meridian that night, the vernal equinox will be rising in the east as the autumnal equinox sets in the west. The June 23 full moon also happens to be a “perigee full moon” (meaning its slightly eccentric orbit happens to bring it closest to earth on the same day it’s full), and it happens to be the closest perigee of the calendar year, too. While these conditions will make the moon measurably (but not noticeably) bigger, a little brighter, and produce higher than usual high tides, it is nothing extraordinary. You’re likely to see all kinds of “Super Moon” posts in the social media, but keep in mind that perigee full moons occur every 14 months and that this one in particular will not get very high in the sky.

Latest Sunrise of the Year

The latest sunrise of the year will occur on January 5 this year...2 full weeks after the shortest day!  The earliest sunset occurred on December 8 even as the days continued to get shorter and on December 21-22 (the Winter Solstice) we experienced the shortest daylight period of the year. Since then the days have been getting longer, even as the sunrise was getting later.

It might seem as if the latest sunrise and earliest sunset should occur on the shortest day, but both the tilt of the earth's axis and it's slightly elliptical orbit work together to speed and slow the sun relative to our clocks, sometimes pushing the daylight period later into the day (as has been happening in the last month), and other times moving the daylight period into the morning in a predictable pattern we call "the equation of time".

The term solstice means sun stops, or sun stands still.  Of course the sun is always moving east to west across our sky, but from late November through mid January, the sun is nearly as far south as gets (it stops moving further south!) - and that's why we see such uniformity in the length of the day....9 hours 20 minutes on 12/8, 9 hours 13 minutes on the solstice, and 9 hours 20 minutes again on 1/4.  It isn't until late February that you'll really notice rapid lengthening of the day.

Earliest Sunset of the Year

December 8, 2011. For the last few years, I've posted this in early December...12/8 marks the earliest sunset of the year! The daylight period is still getting shorter (people who pay attention to these things know that the shortest day is the Winter Solstice around December 21), but not a lot of people can explain tonight's early sunset.  It turns out that the rate at which the Sun travels across the sky is not constant - the tilt of Earth's axis and its elliptical orbit conspire to push the Sun ahead of our clocks, and then slow it down again, twice every year.  Astronomers call the difference between time told by the Sun (apparent solar time) and clock time (mean solar time) the "equation of time".(If you're interested, you can get the sunrise and sunset times for your location at the US Naval Observatory site.)
The chart on the left above, called the analemma, combines the equation of time with the position of the Sun relative to the equator.  Click it for a larger view, and notice that through most of the fall the Sun has been running ahead of the clock, but in December it began to slow dramatically.
It's the Sun slowing down relative to the clock that's moving the daylight period later into the day even as the days get shorter! The worst of winter is still ahead of us, but at least we'll have a little more evening daylight... (the latest sunrise of the year occurs during the first week of January)
This photo composite was made by Tom Matheson over the course of a year, snapping a picture of the Sun at exactly 8 AM (by the clock) each day.  Here is a labeled image of  Tom's photo.

(This blog is an edited  re-post from December 2009 and 2010)