SKY HAPPENINGS

The Sky for July 2011
By Faylene Roth

DAYLENGTH
Earth’s position relative to the sun changes very little as it swings around the far end of its orbit. That explains why the time for sunrise and sunset is slow to change for the first half of the month. Times for sunrise and sunset are calculated for a flat horizon. Actual time depends upon the landscape. Twilight occurs in three stages. Civil twilight provides adequate light for movement and begins about one-half hour before sunrise. Nautical twilight reveals shapes but not detail and begins approximately one hour before sunrise. Astronomical twilight illuminates the sky with a faint glow. It begins nearly two hours before sunrise during most of July. Prolonged twilight is a lingering effect of the Summer Solstice because the northern hemisphere still tilts slightly toward the sun. The same progression applies to dusk.

MOON HAPPENINGS
July 1 – New Moon occurs at 2:54am.
July 8 – First Quarter Moon sets soon after midnight.
July 15 – Full Moon occurs at 12:40am. (Rises July 14 at 8:20pm; July 15 at 8:58pm.)
July 23 – Last Quarter Moon rises soon after midnight.
July 30 – New Moon occurs at 12:40pm.

APHELION
Earth reaches its farthest distance from the sun at 3:00am on July 4. It is a coincidence that the earth reaches this point, called aphelion, so soon after the summer solstice. Aphelion and perihelion mark the far and near distances from the sun during the earth’s annual solar orbit. The period from one aphelion to the next is about 25 minutes longer than the calendar year (or seasonal year from one solstice to the next). This difference amounts to a delay of about one day every 58 years.
Three major cycles affect the position of Earth relative to the sun. The abovementioned change in the period between one aphelion and the next is primarily due to precession of the equinoxes. Precession is due to a wobble in the direction of the earth’s axis (celestial north pole) similar to the spin of a toy top. Over a 21,000 year cycle the direction of the earth’s axis will move the north celestial pole in a counter-clockwise direction from Polaris (Ursa Minor), through Deneb (Cygnus), Vega (Lyra), and Thuban (Draco). The effect of precession is to slowly shift the seasons to later in the year.
Obliquity of the tilt of the earth’s axis is another cycle. Over a period of about 41,000 years the tilt of the earth varies a few degrees on either side of its current 23.4 degree tilt. It is the tilt of the axis that causes the earth’s seasonal changes
A final cycle tracks the changes in eccentricity of the earth’s orbit. Over 100,000 years, the elliptical orbit varies from nearly circular to three times more elliptical than its current orbit. A more elliptical orbit would stretch the difference between perihelion and aphelion from its current three percent difference to as much as a nine percent difference.
For information on how these cycles may interact to affect Earth’s climate, read about Milankovitch cycles.

METEOR EVENTS
Activity from the Alpha Capricornid Meteor Shower occurs July 23-30. Its radiant is the constellation Capricornus which follows Sagittarius into the southeastern sky near midnight. Fireballs are frequently produced from these bright yellow meteors. The Southern Delta Aquarid Meteor Shower begins activity around July 18. The Aquarids produce up to 20 meteors per hour at its peak on the night of July 28/29. Its radiant is the constellation Aquarius which rises after Capricornus. Meteor activity continues for a week after peak activity. One of the best meteor showers of the year is the Perseids. Activity begins during the last half of July but will not peak until mid-August. Look beyond Cassiopeia for the constellation Perseus which is the radiant for these meteors. A waning new moon provides excellent viewing conditions. Best viewing is after midnight.

Note: Hold your hand at arm’s length to measure apparent distances in the sky. The width of the little finger approximates 1.5 degrees. Middle, ring, and little finger touching represent about 5 degrees. The width of a fist is about 10 degrees. The fist with the thumb extended at a right angle equals 15 degrees. The hand stretched from thumb to little finger approximates 20-25 degrees. The diameter of both the full moon and the sun spans only 0.5 degree. Adjust for the size of your hand.

Primary Sources: USGS; U.S. Naval Observatory;
Your Sky at http://www.fourmilab.ch/yoursky/

To find out when the space shuttle and International Space Station are visible
from your location, go to: http://spaceflight.nasa.gov/realdata/sightings/index.html and click on Sighting Opportunities.

VISIBLE PLANETS
Jupiter - The bright planet high in the eastern sky during morning twilight is Jupiter. It now rises a couple hours after midnight on the Pisces/Aries border. (Magnitude -2.4)

Mars - Look for Mars on the eastern horizon about two hours after Jupiter rises which is about two hours before dawn becomes apparent in the morning sky. The red planet shares the sky with the red-giant star, Aldebaran, in the V of Taurus. Mars appears about six degrees north of Aldebaran. (Magnitude +1.4)

Mercury - Find Mercury in the NNW skies about 45 minutes after sunset. On July 2 it appears about five degrees above a waxing crescent moon. By month’s end Mercury will be in Leo, too low on the western horizon to be easily seen. (Magnitude +0.5)

Saturn - The ringed planet has moved into the western sky. It remains in Virgo near the double-star Porrima. (Magnitude +0.9)

Venus - The brightest of planets is the last of the three morning planets to rise—less than an hour before the sun. It will be too low on our horizon to be easily seen . (Magnitude -3.9)

Note: Apparent magnitude values range from -4 to +6 for most planets and visible stars. The lower the value the brighter the object. A decrease of 1.0 magnitude is 2.5 times brighter.

The Summer Triangle hangs high in the eastern sky, formed by 0 magnitude star Vega (Lyra) and 1st magnitude stars Deneb (Cygnus) and Altair (Aquila). Overhead is Hercules--his squarish body halfway between Arcturus (Bootes) and Vega.

Hold the star chart high above your head and match the compass directions to the direction you are facing. Adjust the star chart by orienting Ursa Major (Big Dipper) to match its position in the sky.

The star chart approximates the sky from astronomical twilight to midnight. As the night and month progresses, the constellations shift toward the northwest. The celestial equator is measured in hours (h).
The ecliptic is measured in degrees.