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U.S. Naval Observator Astronomical Information center

How do I convert Universal Time to my time?

Sky Tonight—May 16, Will you see a fuller moon tonight … or tomorrow?

admin — Mon, 16 May 2011 00:45:10 +0000

Courtesy of EarthSky
A Clear Voice for Science
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Photo from yourdon’s photostream

Your calendar probably says that full moon is tomorrow, Tuesday, May 17, 2011. However – depending on where you live and what time you look – you might see a fuller moon tonight.

That is true for us in North America. Full moon falls at or shortly before dawn on May 17 for us. So tonight’s moon is slightly more full than the moon tomorrow night – even though the calendar says that tomorrow is the full moon. This happens pretty frequently because astronomical almanacs typically tell the full moon time in what’s called Universal Time – standard clock time at Greenwich, England. For us in the Americas, Universal Time (UTC) or Greenwich Mean Time (GMT) is some hours ahead of our clocks.

At this full moon instant, the moon will swing most directly opposite the sun for the month. It will be the crest of the moon’s full phase. At every full moon – for half the world – the full moon is under the horizon during the daylight hours when it is most full. This month, that is the case for Europe, Africa, most of Asia, and most of South America. If you are in one of those places, the moon will appear about equally full tonight and tomorrow night.

In contrast, if you are across the International Dateline from the Americas, the moon will be most full after sunset tomorrow. That is you if you are in Japan, the Philippines, Australia, or New Zealand.

If you are going to plan a full moon party, it is best to plan it for the day or two before the calendar says the moon will be full. Then you can be sure of finding a bright, nearly full moon in your sky when the sun goes down. That is because a full moon always rises at sunset, and the moon rises later every day as it orbits Earth. Therefore, the day after full moon, you could wait nearly an hour after sunset to see a moonrise.

To know when the moon is fullest for you, you have to convert Universal Time to the clock time in your own time zone.

The world map below helps you to envision the day and night sides of Earth as the moon turns full on May 17 at 11:09 Universal Time.

: Earth's day and night sides at the instant of full moon (2011 May 17 at 11:09 Universal Time)

No matter where you live, however, look for a full-looking moon to rise over your eastern horizon after sunset tonight and to sit low in the west before sunrise tomorrow. Watch the big, round moon light up the night from dusk until dawn!

World Map, courtesy of John Walker’s Earth and Moon Viewer.

By Bruce McClure

U.S. Naval Observator Astronomical Information center

Understanding moon phases

Sky Tonight—May 15, Earth shadow, Belt of Venus in east after sunset

admin — Sun, 15 May 2011 05:05:05 +0000

Courtesy of EarthSky
A Clear Voice for Science
Visit EarthSky at
www.EarthSky.org

Here’s a lovely photo of the Earth’s shadow, which can be seen any clear evening ascending in the eastern sky at exactly the same rate that the sun sets below the western horizon. The shadow is a deep blue-grey – darker than the blue of the twilight sky.

The pink band above the shadow is called the Belt of Venus.

The shadow of the Earth is big. You might have to turn your head to see the whole thing. The shadow is curved, in just the same way that the whole Earth is curved. Earth’s shadow extends hundreds of thousands of miles into space, so far that it can touch the moon. Whenever that happens, there is an eclipse of the moon.

Check out Earth’s shadow – in the east at sunset – next time you have a clear sky. I often see it while out on the streets of my town as the sun is setting.

By the way, this image shows more or less the same moon phase that you will see tonight. It will be a waxing gibbous moon that will be visible in the east after sunset this evening. Full moon will come on May 17, 2011.

This image is the full moon at the Woomera missile range in Australia. Used with permission.

Understanding waxing gibbous moon

Can you tell me the full moon names?

By Deborah Byrd

U.S. Naval Observator Astronomical Information center

Sky Tonight—May 14, Bright star near moon is Spica

admin — Sat, 14 May 2011 05:14:21 +0000

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A Clear Voice for Science
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The waxing gibbous moon glides past the star Spica in the constellation Virgo this evening. It passes relatively close to Spica for a day or two each month that Spica is visible in our night sky. It does the same with other bright stars such as Regulus in the constellation Leo, Antares in the constellation Scorpius and Aldebaran in the constellation Taurus.

But have you ever heard of the moon near Polaris the North Star? If someone tells you they saw the moon near Polaris, you will know that cannot be so. The moon has a set path in the sky, approximately the same path as that followed by the sun each day. Polaris is far to the north on the sky’s dome, in a part of the sky the moon never visits.

In reality, the moon’s nearness to Spica tonight is just a line-of-sight illusion. The moon never gets close to Spica in a true sense because the moon is only about one light-second from Earth, while Spica is 260 light-years away.

Spica is the 15th or 16th brightest star in the sky (it’s neck-and-neck with Antares). Spica is a close double star, and both stars in the Spica system are much hotter and brighter than our sun. In fact, there is some evidence that there are as many as three more smaller, fainter stars in the Spica system.

By Larry Sessions

U.S. Naval Observator Astronomical Information center

Sky Tonight—May 13, Moon near golden Saturn

admin — Fri, 13 May 2011 04:52:22 +0000

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A Clear Voice for Science
Visit EarthSky at
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You’ll find the moon in a waxing gibbous phase this evening – more than half lighted but less than full – in the sky from sunset until after midnight.

Tonight’s moon will be visible near a bright object in our sky. It is not a star, but a planet – Saturn, planet of the rings. Given clear skies, almost everyone around the world can see the moon and Saturn near each other this evening.

Earth passed between Saturn and the sun on April 3 of this year. That was Saturn’s yearly opposition, and it means that we are farther from Saturn now than we were six weeks ago. We are now racing ahead of Saturn in Earth’s smaller, faster orbit around the sun. So, Saturn is steadily appearing dimmer on our sky’s dome, and it is in our sky for fewer hours of each night.

There is a bright star near the moon and Saturn, too, tonight. It is Spica; the brightest light in the constellation Virgo the Maiden. Spica is the 15th brightest star in all the heavens.

Saturn, like all planets, will be shining with a steadier light than the twinkling stars, and it will be the closer bright light to the moon tonight.

In contrast to the moon, Saturn moves at a snail’s pace across the dome of night. By tomorrow night, the moon will have moved on, pursuing its endless orbit around Earth.

By Deborah Byrd

U.S. Naval Observator Astronomical Information center

Sky Tonight—May 12, How to see the green flash

admin — Thu, 12 May 2011 02:38:33 +0000

Courtesy of EarthSky
A Clear Voice for Science
Visit EarthSky at
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The image today is a classic image of a detached green flash, seen at sunset.

You can see green flashes with the eye, sometimes, if you are looking toward a very clear horizon. You must be looking just at sunset, at the last moment before the sun disappears below the horizon and you have to be careful not to look too soon. Wait until just the thinnest rim of the sun appears above the horizon. If you look too soon, the light of the sunset will dazzle (or damage) your eyes, and you will miss that day’s green flash.

There are many different types of green flash. Some describe a streak or ray of the color green … like a green flame shooting up from the sunrise or sunset horizon. The most common green flash, which many people describe, is a flash of the color green seen when the sun is nearly entirely below the horizon.

You need a distant horizon to see any of these phenomena, and you need a distinct edge to the horizon. These green flashes, streaks, and rays are often seen over the ocean – but you can see them over land, too, if your horizon is far enough away. Pollution or haze on the horizon will hide this instantaneous flash of the color green.

If you are interested in green flashes, Andrew Young’s green flash page is great. He also has a page of links to pictures of green flashes taken by people from around the globe.

Here is another good article about the green flash by Peter Michaud.

Here is a rather subtle green flash video. I had to watch it several times to convince myself I saw any green.

By Deborah Byrd

U.S. Naval Observator Astronomical Information center

Sky Tonight—May 11, Moon brushes the belly of Leo

admin — Tue, 10 May 2011 13:42:18 +0000

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A Clear Voice for Science
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The moon is waxing larger in the evening sky each night. The first quarter phase happened yesterday, and now it is a waxing gibbous moon.

Tonight’s moon is near the star Regulus and brushing up against the belly of the constellation Leo the Lion throughout the evening.

Regulus is the brightest star in the constellation Leo the Lion. It dots the backward question mark of stars dubbed the Sickle. The Sickle is what has called an “asterism” – not a whole constellation but simply a noticeable pattern of stars on the sky’s dome. The Sickle outlines the head and mane of Leo the Lion, and the triangle of stars to the left of the Sickle depicts the Lion’s hindquarters and tail.

Regulus sits almost squarely on the ecliptic – the sun’s path through the constellations of the zodiac. Annually, the sun passes in front of Leo from about August 10 to September 15, and pairs with Regulus on or near August 23. On this date, Regulus rises with the sun, shines with the sun at midday, and sets with the sun. Six months later, in February, Regulus will be opposite the sun in our sky, lighting the nighttime from dusk until dawn.

Tonight, watch for the moon to brush against the belly of the mighty Lion!

By EarthSky

U.S. Naval Observator Astronomical Information center

Sky Tonight—May 10, First quarter moon shows a “flat” edge

admin — Tue, 10 May 2011 13:37:42 +0000

Courtesy of EarthSky
A Clear Voice for Science
Visit EarthSky at
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As the moon lights up the sky this evening, everyone will see the moon fairly close to Regulus, the brightest star in the constellation Leo. The moon is not flat, but tonight’s first quarter moon does appear to have a “flat” edge to it.

The apparently flat edge of the moon at the quarter phase is called the “terminator.” No, that is not a reference to a Schwarzenegger sci-fi movie. Instead, it refers to the fact that sunlight “terminates” or ends at this line.

In other words, it is the sunrise line on the moon. In the lit portion, it is daytime, and on the other side of the line, it is night. In reality, it is not correct to refer to the “dark side of the moon” as any permanent place, because with a constantly moving terminator, any location on the moon is in the dark half of the time, and in the daylight half of the time.

Sometimes astronomers refer to tonight’s moon phase as the “letter D” moon because of its apparent shape. It appears with a similarly “flat” edge at last quarter phase, but fewer people notice the backwards “D” because of the timing.

The moon’s phases are a result of the moon’s changing position relative to the sun and Earth. Check out this really great Flash demonstration: How to simulate lunar phases.

Before sunrise tomorrow for binoculars:

With an unobstructed eastern horizon and binoculars, you might be able to spot the Venus/Jupiter conjunction before sunrise tomorrow (May 11). Look for these two worlds very low in the east about 60 to 30 minutes before sunrise. Venus, the brighter of the two, ranks as the third brightest celestial body to light the heavens, after the sun and moon. Jupiter shines as the fourth brightest. This conjunction will be much easier to see from the southern hemisphere, because these planets rise sooner before the sun in that part of the world.

By Larry Sessions

U.S. Naval Observator Astronomical Information center

Looking for a sky almanac? EarthSky recommends …

Sky Tonight—May 6, Mercury at greatest morning elongation May 7

admin — Fri, 06 May 2011 04:46:40 +0000

Courtesy of EarthSky
A Clear Voice for Science
Visit EarthSky at
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Mercury, the solar system’s innermost planet, reaches its greatest western elongation from the sun on Saturday, May 7. Because Mercury is farthest west of the sun at present, this world now rises into the morning sky before sunrise – but how much before depends on where you live on the globe.

The farther north you live, the closer Mercury rises to sunrise. The farther south you live, the greater the period of time between the rising of Mercury and the sun. For instance, at mid-northern latitudes, Mercury comes up about 45 minutes before sunrise. Yet, at temperate latitudes in the southern hemisphere – as in southern Australia – Mercury rises two hours or more before the sun.

Do not mistake the dazzling planet Venus for Mercury. Mercury, though as bright as a first-magnitude star, pales next to Venus, which outshines Mercury by nearly 60 times. If you see Venus but not Mercury, aim binoculars at Venus to spot Mercury nearby.

Three planets will fit within the same binocular field before sunrise for well over a week: Venus, Mercury, and Jupiter. Venus is the brightest of the pack, followed by Jupiter and then Mercury.

Mars, the faintest of the four morning planets, will share the same binocular field with Jupiter for the next few days. From the northern hemisphere, however, Mars will be exceedingly hard to see, with or without binoculars.

Before sunrise, use brilliant Venus to find fainter Mercury, which reaches its greatest morning elongation on Saturday, May 7.

Animation of May 2011 morning planets

By Bruce McClure

U.S. Naval Observator Astronomical Information center

Why does the Eta Aquarid meteor shower favor the southern hemisphere?

Sky Tonight—May 5, Eta Aquarid meteor shower best before dawn on May 6

admin — Thu, 05 May 2011 04:26:55 +0000

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A Clear Voice for Science
Visit EarthSky at
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Photo courtesy of tonynetone’s photostream

The annual Eta Aquarid meteor shower is expected to produce the most meteors before dawn tomorrow (Friday, May 6). But – because these meteors have a relatively broad maximum – you can look for some Eta Aquarids before dawn on Saturday, too.

Planets visible at dawn: Venus and possibly Mercury and Jupiter

The Eta Aquarid meteors are strictly for night owls or early risers. This shower does not start until around 2 or 3 a.m. The meteors are few and far between at that hour, but the wee hours are a time for catching earth-grazing meteors. An earth-grazer is a long, slow, colorful meteor that horizontally streaks the sky.

The closer to dawn, the more Eta Aquarid meteors you are likely to see. These meteors are extremely fast and often bright, striking Earth’s atmosphere at 66 kilometers – about 41 miles – per second. Many of the brighter meteors leave persistent trains – glowing ionized gas trails – for a few moments after their fiery plunge.

To recap, the Eta Aquarid meteors will probably be at their best during the predawn hours Friday. If you plan to watch them, you will want a dark, wide-open sky – a reclining lawn chair – and for your feet to point in a general eastward direction. That is where the radiant point for this shower – in the constellation Aquarius the Water Bearer – will rise. At mid-northern latitudes, expect 10 to 20 meteors per hour. South of the equator, you might see twice that many!

Find the radiant of the Eta Aquarid meteor shower

EarthSky’s meteor guide for 2011

By Bruce McClure

U.S. Naval Observator Astronomical Information center