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Sky Data

Sky Toward Sagittarius

Stars and deep sky objects in the vicinity of Sagittarius.

Credit: W. Keel (U. Alabama in Tuscaloosa), Cerro Tololo, Chile

Monday July 4th Earth is at maximum distance from the Sun, a juxtaposition called aphelion. At 11 a.m. on Independence Day the Earth-Sun distance is about 94.5 million miles. This compares to the situation at perihelion, when Earth is at its minimum solar distance of 91.4 million miles. Perihelion last occurred the afternoon of January 3rd. On that date the daily average temperature in New York City is 33 degrees Fahrenheit. Average temperature on July 4th is 75 degrees. Clearly, distance from the Sun doesn’t affect our seasonal temperature variations. Seasons are caused by changing angles between the direction of Earth’s axis and the direction toward the Sun. Maximum inclination of the axis toward the Sun occurred on June 21st. However local average temperatures in the New York City area continue to rise slightly, despite diminishing sunlight, until about the third week of July due to residual heat retained on land and in surrounding waters.

Friday July 1st, civil twilight begins at 4:55 a.m. and sunrise is at 5:28. That evening the sun sets at 8:31 p.m. and civil twilight concludes at 9:04.

On the last day of July civil twilight begins at 5:20 with sunrise at 5:51 a.m. That evening sunset occurs at 8:12 p.m. and civil twilight ends at 8:43.

During July average overnight temperatures rise from 67 degrees Fahrenheit on the first of the month, peak around the 21th, and then drop slightly to 69 degrees by the 31st.

Principal Phases of the Moon this month are:

New Moon July 1
First Quarter July 8
Full Moon July 15
Last Quarter July 23
New Moon July 30

Sky Cast

Jupiter and Mars are visible in the predawn eastern sky throughout July. At the start of the month Venus is low in the east-northeast during morning twilight. It continues to move closer to the direction of the Sun and by mid-month is lost in solar glare.

The evening of Friday July 1st, Mercury is in the west-northwest at an altitude of about 8 degrees of arc, as evening twilight fades around 9 p.m. At that time Saturn is 40 degrees above the southwestern horizon. This sixth planet from the Sun shines with a yellowish glow and is currently at visual magnitude 0.87.

If you have an unobstructed view toward the west at about 8:50 p.m. on Saturday July 2nd, try looking for the very thin crescent moon just five degrees above the horizon toward west-northwest. It is then six degrees of arc to the lower left of Mercury. With so many buildings in our area, best chances for seeing objects low in the west are when looking across relatively wide bodies of open land or water from the highest elevation possible. It’s a good idea to scout such sites well in advance of proposed observations. Use the length of your fist with arm fully extended to approximate a ten degree span. Each finger width then represents about two degrees of arc.

At the conclusion of evening twilight Sunday July 3rd, the crescent moon appears low in the western sky, about 14 degrees to the left of Mercury.

The evening of July 7th towards the southwest, a wide crescent moon is about eight degrees from Saturn. During July Saturn provides some of the loveliest views available to telescopic observers. If you haven’t used your telescope for a while, practice aligning the instrument’s sights toward buildings or trees during day time. Such eye-hand-telescope activity makes finding elusive nighttime points of light much easier. Just remember when sighting telescopic objects during daytime, NEVER point the telescope anywhere near the Sun.

Friday July 8th the first quarter moon is at the southern portion of the constellation Virgo, four degrees from first magnitude star Spica.

Around ten p.m. during the second week of July, red supergiant star Antares, brightest in the constellation Scorpius, is near the celestial meridian dividing eastern and western halves of the sky. On the evening of Monday the 11th, a bright waxing gibbous moon joins Antares by the meridian.

Friday July 15th the Full Moon is in the western part of the constellation Capricorn.

Saturday July 16th the NASA Dawn spacecraft settles into an orbit around Vesta, the second largest object in the main asteroid belt. Although, after the promotion of Ceres to dwarf planet status, Vesta is now classified as the largest asteroid, it is also considered to be a protoplanet, formed early in the development of the solar system 4.5 billion years ago. It potentially might have become covered with additional rocky layers, perhaps becoming a major planet, but disruptive forces of Jupiter’s huge gravity influence prevented that scenario. On July 16th the Dawn craft will be about 9,900 miles from Vesta, both being 110 million miles from Earth. It will mark the start of a yearlong visit during which Dawn will survey the 330 mile wide asteroid from altitudes as low as 120 miles. Results are expected to provide extensive information about conditions in the early solar system.

Ceres and Vesta

Composite images of dwarf planet Ceres and asteroid Vesta taken with the Hubble Space Telescope in 2007.

Credit: NASA, ESA, L.McFadden, J.Y.Li (UMCP), M.Mutchler, Z.Levay (STScI), P.Thomas (Cornell), J.Parker, E.Young (SwRI), C.Russell, B.Schmidt (UCLA)

You might like to use binoculars and look for 6th magnitude Vesta the night of July 16-17th. That Saturday evening the asteroid rises shortly after 10 p.m. Begin looking after 11 p.m. when it has moved away from the horizon. Vesta will be about nine degrees southeast of the moon in the constellation Capricornus. Vesta’s location that night will be about halfway between the fourth magnitude stars gamma and zeta Capricorni, to the west (right) of a line imagined to connect these stars. At about the same location, but on the eastern (left) side of that line is the 6th magnitude star 37 Capricorni. It can be identified by its proximity to a fainter 7th magnitude star located just 1/3 of a moon diameter to the south. Vesta reaches an altitude of 17 degrees above the southeast horizon at midnight.


Dawn Mission image of asteroid Vesta taken June 20, 2011.


By the end of July Vesta and its new human-made satellite will have moved to a position two degrees west of zeta Capricorni. After its yearlong visit, the Dawn spacecraft will leave Vesta and proceed to a 2015 encounter with Ceres, a 600 mile wide object originally classified as a planet after its discovery in 1801, later considered the largest asteroid, but now called a dwarf planet.

This month ninth magnitude Ceres is in the constellation Cetus, about four degrees west of considerably fainter 16th magnitude comet Temple 1. Six years ago this comet was spectacularly studied by the Deep Impact mission that purposely crashed a large copper impactor into that icy body’s surface on the Fourth of July 2005.

The following images show that since June 20th our view of Vesta from the approaching Dawn mission camera exceeds in clarity the best picture of this asteroid provided by the Hubble Space Telescope from its Earth orbit.

During early morning hours of Saturday July 23rd, the waning third quarter moon is about eight degrees to the upper right of Jupiter which shines at visual magnitude minus 2.4 in the constellation Aries. The next evening the moon is seven degrees to the upper left of the big planet.

During the predawn of Monday July 25th the waning crescent moon is about 4 degrees southwest of the Pleiades star cluster. The combination is a fine sight in binoculars.

Sky Lore

During July the magnificent Band of the Milky Way arches across the sky from northwest to south. Along its path many constellations feature objects of interest. Stars in Perseus, Cassiopeia, Cygnus, Aquila, Scutum, Sagittarius, and Scorpius encompass the equator of the Galactic Band. The beautiful double star Alberio in Cygnus; open star cluster M11, known as the Wild Duck Cluster in Scutum; open clusters M6 and M7 in Sagittarius; and globular cluster M3 in Scorpius are impressive features of the Milky Way zone. In order to fully appreciate the sweep of the Milky Way a dark sky observation site is needed.

Cygnus was traditionally associated with a bird in Middle Eastern and classical sky traditions. Brightest star in this constellation is first magnitude Deneb, one of the stars that mark a corner of the Summer Triangle asterism. Deneb’s estimated distance is about 3,000 light years, making this blue-white supergiant one of the most distant stars we can see with unaided eyes. Lovely blue and yellow components of double star Alberio mark the Swan’s head. Also in this constellation, the black hole associated with x-ray source Cygnus X-1 was the first of its kind to be identified.

First magnitude Altair, at a distance of 17 light years, indicates a second Summer Triangle corner. This blue-white main sequence object is at the center of a close group of three stars that also includes third magnitude Tarazed and fourth magnitude Alshain. All are in the constellation Aquila, the Eagle.

The third and brightest corner star of the Summer Triangle is Vega in the constellation Lyra, the lyre. Vega has a visual magnitude of zero and is a blue-white main sequence star. Main sequence indicates a star in its prime, producing light and heat by thermonuclear fusion of hydrogen to helium in its core. Vega’s distance from the solar system is 25 light years.

Along the Milky Way extending to the southwest of Aquila is a small constellation named Scutum, the shield. Near its eastern border is the open star cluster formally known as Messier 11, popularly called the “Wild Duck” cluster. Open star clusters get their name from their lack of sufficient numbers capable of providing the mutual gravitational attraction needed to prevent the group’s eventual dissolution. However, such a cluster’s demise may stretch over a period of hundreds of millions of years.

Crossing Scutum’s southern boundary brings us to stars of Sagittarius the Archer. Sagittarius includes wide sections of the Milky Way’s light band and the direction to the Galaxy’s center. The brightest stars of Sagittarius trace a figure that many associate with the shape of a teapot. Concentrated in the immediate area of Sagittarius are numerous Galactic objects far from the Sun’s neighborhood including a bevy of globular star clusters and star forming nebulae. Near the teapot’s spout is the direction toward the super massive black hole located about 26,000 light years from us at the center of the Milky Way Galaxy.

West of Sagittarius is a dramatic pattern of stars tracing the celestial scorpion, Scorpius. Brightest star in this constellation is red supergiant Antares. It is about 600 light years from the solar system.

You would enjoy observing from a dark sky site where the Band of the Milky Way is clearly visible. Absence of moonlight also would be required. Use your browser and go to Attilla Danko’s “Clear Sky Clock” website. It provides information including weather forecasts of interest to sky watchers, and a map showing relative darkness of tri-state area sites about 120 miles from Manhattan. Note – unless you have access to a seaworthy boat, the nearest dark locales in the New York City area are in the Catskill Mountain region. For darkness conditions, see Clear Dark Sky.

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