Skip to main content (Press Enter).
Official Website of the United States Navy
Mission & Vision
Naval Oceanographic Office
Fleet Numerical Meteorology & Oceanography Center
United States Naval Observatory
News from the Naval Observatory
Earth Orientation Department
USNO Earth Orientation Products
Contents of Bulletin A
USNO GPS Products
GPS User Information
USNO VLBI-based Products
VLBI Correlator Data
VLBI-based Earth Orientation Parameters (EOP)
Earth Orientation Information Center
General Information about Earth Orientation
Frequently Asked Questions About Earth Orientation
What Is Earth Orientation?
What Is Polar Motion?
What does the Earth rotation coordinate measure?
What is the Celestial Pole Offset?
What is a Leap Second?
How do we measure Earth Orientation?
What causes variations in the Earth's orientation?
Who uses Earth orientation information?
Publications About Earth Orientation Products
Earth Orientation Software
Precise Time Department
The USNO Master Clock
The USNO Master Clock
Time Dissemination at the USNO
USNO Alternate Master Clock (AMC)
Cesium Atomic Clocks
Hydrogen Masers at the USNO
Rubidium Fountain Clocks
USNO Time Scales
International Time Scales and the BIPM
Definitions of Systems of Time
Global Positioning System
Global Positioning System Overview
USNO GPS Data Categories Explanation
CGGTTS Data Format
USNO GPS Time Transfer
GPS Information: SA, DGPS, Leap Seconds, etc.
GPS Week Number Rollover
GPS Timing Data and Information
USNO Format Explanation
USNO Computer Display Clocks
Two-Way Satellite Time Transfer (TWSTT)
Network Time Protocol (NTP)
US Eastern Time Zone NTP Servers
US Mountain Time Zone Servers
DoD Customer Servers
Astronomical Applications Department
Astronomical Information Center
Astronomical Data and Litigation
Celestial Reference Frame Department
Naval Oceanography Operations Command
Fleet Weather Center - Norfolk
Fleet Weather Center - San Diego
Public Use of Limitations
Work With Us
United States of America Department of the Navy Seal
The Sky This Week
U.S. Naval Observatory's Weekly Blog
Something For Everyone on Astronomy Day
by Geoff Chester, USNO Public Affairs
11 May 2021
Venus (above clouds) and Mercury, imaged 2021 May 9, 00:45 UT from Mollusk, Virginia
with a Canon EOS Rebel SL2 DSLR. HDR composite of 3 exposures, 34mm f/8, ISO 200.
The Moon returns to grace the evening sky this week, waxing to First Quarter phase on the 19th at 3:12 pm Eastern Daylight Time. This week offers one of the best opportunities to spot a very slender lunar crescent. On the evening of the 12th the 1.4 percent sliver of the Moon will be located just to the left of the bright planet Venus. At 8:45 pm you should be able to spot Venus about five degrees above the west-northwest horizon. Luna’s crescent should become apparent at around that time as well. Find a good site with an unobstructed view to the west to find this interesting pair. On the following evening the Moon should be easy to spot. Look a few degrees to the right of the Moon to find the elusive planet Mercury. On the night of the 15th you will find a dimming Mars just over two degrees northeast of Luna.
May 15th marks the annual spring observance of International Astronomy Day. This event, first celebrated in California in 1973, has grown into a world-wide effort on behalf of amateur astronomers to involve the public in their fascinating hobby. It is now held twice a year in the spring and fall on a Saturday near the time of the First Quarter Moon. Here in the Washington metropolitan area it has been promoted and staged by the Northern Virginia Astronomy Club (NOVAC) for many years, and takes place at C.M. Crockett Park near Midland, Virginia from 5:00 until 11:00 pm EDT. Cub members will be on hand to discuss telescopes, observing, and imaging techniques, and telescopes will be set up for visitors to view through. The event is open to the public, but non-Fauquier County residents must pay a nominal park fee. COVID protocols will be observed, so face masks will be required of all participants. More information is available on the NOVAC website.
This is one of those weeks where I hope for clear skies every night, as there is something for every level of interest to enjoy. The Moon coursing her way past the evening’s planets offers early evening skywatchers the treat of looking at the battered surface of another world nearly a quarter million miles away. Luna is a perfect target for binoculars and small telescopes, and as her crescent thickens from night to night new landscapes sweep into view. Luna sets by the late evening through most of the week, leaving a few hours of dark skies to view some of the other wonders of the springtime sky. At this time of year the “deep sky” is dominated by faint smudges of light that are the signatures of distant galaxies scattered among the stars of Ursa Major, Leo, Boötes, Coma Berenices, and Virgo. A four-inch telescope will reveal dozens of these “island universes”, most of which are tens of millions of light-years distant. It is also a time to start seeing so-called globular star clusters, which resemble luminous spherical puffs of smoke in the eyepiece. One of the most spectacular of these objects may be found in the constellation of Hercules, which is high in the east by 11:00 pm. The
Great Hercules Cluster
resolves into a myriad of faint stars in my 4-inch telescope, and increasing the aperture to 8-inches gives it the appearance of a luminous dandelion seed crown. These clusters are remnants of the cores of ancient dwarf galaxies that formed with our Milky Way some 13 billion years ago. Our much larger galaxy has stripped these star swarms of their star-forming dust and gas, leaving their primordial spherical cores to slowly orbit our galaxy’s massive central bulge.
As mentioned earlier, the Moon passes the innermost planets Venus and Mercury early in the week. Venus has just emerged from conjunction with the Sun and is gradually climbing above the western horizon. Elusive Mercury will reach his greatest elongation east of the Sun on the 17th, so he is at his best in the early evening sky right now and for the next couple of weeks.
Mars is plodding relentlessly eastward through the stars of Gemini, and he, too, gets a visit from the Moon on the 15th. The red planet now shows a tiny pink disc in the telescope, and he has faded to nearly second magnitude. He gradually pulls away from the third magnitude star Mebsuta as the week progresses.
Jupiter and Saturn still rise in the wee hours of the morning and are still best seen well before sunrise. You will find them on the southeastern sky as morning twilight begins. At this time both planets are high enough to give a good view in the telescope.
DoD Accessibility/Section 508
No Fear Act
Plain Writing Act
Veterans Crisis Line
DoD Safe Helpline
Commander, Naval Meteorology and Oceanography Command | 1100 Balch Blvd. | Stennis Space Center, Mississippi 39529
Official U.S. Navy Website