Go Stargazing! May Edition

Published by James on May 4, 2010 at 10:16 am in Astronomy

Saturn is now in the south southeast at dusk. We are seeing its rings a little more edge on than earlier in the year. In fact, Saturn’s rings won’t be this edge-on to us for another 15 years. Saturn, like Earth, is tilted on its axis (at 26.7 degrees, Saturn’s tilt is a little greater than Earth’s). Twice per Saturn orbit, then, about every 15 years, Saturn has equinoxes where the sun is aligned with Saturn’s equator. Since the rings orbit the equator, this puts the sun (and the Earth) in Saturn’s ring plane. Earth was exactly in Saturn’s ring plane on September 3, 2009 when Saturn was also on the far side of the sun and hard for us to see. This month, Earth again approaches (but will not cross) Saturn’s ring plane. That’s why the rings appear so thin in telescopes now. Learn more about the rings of Saturn in my latest blog post.

Venus keeps getting higher in the evening sky during May. Face west at dusk and look for a point of light that outshines everything in the sky but the sun and the moon.

Mars is very high in the evening sky, although not as bright as it was in winter. Since January 29, Earth has been pulling ahead of Mars on its faster orbit. As a result, Mars gets slightly dimmer each night for the rest of 2010. However, during May, Mars remains brighter than average, and thus remains easy to see. Look high in the west at dusk for a reddish point of light.

Jupiter is low in the southeast at dawn this month. Look for it low in twilight as day begins to break. It will be higher in the southeast by the end of the month.

In May, you can watch as the Dog Days begin! We are in the Dog Days when the Dogs have vanished from the sky. As May begins, Orion, the Hunter is clearly visible due west right after sunset. To his left, aligned with Orion’s belt, is Sirius, the Dog Star, the brightest star we see at night. Forming a triangle with Sirius and Orion’s brightest star Betelgeuse is Procyon, the Little Dog Star. Throughout May, watch as Sirius appears slightly lower and lower to the horizon each night, until it is gone by May 31. By mid-June, Procyon is gone as well. When the Dogs are up only in the day, we’re in the Dog Days.

Meanwhile, spring stars are high in the south and east. A distinct backwards question mark shape outlines the mane and forepaws of Leo, the Lion. Three stars forming a right triangle rise underneath; they mark Leo’s hindquarters. The Big Dipper is as high as it ever gets in the north at dusk. You can extend the curve of its handle to ‘arc to Arcturus’ and then ‘speed on to Spica’. These stars high in the east and southeast, respectively, by dusk tonight.

星空下的汗腾格里峰 / Mt. Khan Tengri under Galaxy

photo credit: livepine

As Orion and Sirius set, the plane of the Milky Way largely coincides with the horizon. (At Houston’s latitude, the two planes are off by less than three degrees). We are therefore looking straight out of the Milky Way plane when we look up early on a May evening. Thus May evenings have fewer bright stars, as most of the brightest stars in the Milky Way plane are ringing the horizon.

Moon Phases in May 2010:

Last Quarter May 5, 11:15 pm

New May 13, 8:05 am

First Quarter May 20, 6:43 pm

Full May 27, 6:07 pm

Go Stargazing! May Edition

Published by James on May 4, 2009 at 4:36 pm in Astronomy

The Pleiades

photo credit: Evilnick

Have you ever wondered when the ‘Dog Days’ begin? The term is based on Sirius, the Dog Star. In the time of the Ancient Egyptians, Sirius rose right before sunrise at the summer solstice, after having been invisible for about 2.5 months. Egyptians began their calendar years with this event, and also considered Sirius a herald of summer. Ancient Greeks and Romans believed that Sirius, as the brightest star in the night sky, helped cause the hot and oppressive weather of summer when it rose with the Sun.

Because Earth wobbles as it spins, we no longer see the constellations in quite the same places the ancients did (this is called precession of the equinoxes.) As a result, Sirius now rises with the Sun in mid-August. So that the term refers to roughly the same time of year, modern folklorists (such as the Farmer’s Almanac) take the Dog Days to end with the reappearance of Sirius and to begin when Sirius leaves the evening sky. May is the month to watch Sirius slowly go away. It’s easily visible in the southwest tonight and next week. As May continues, notice Sirius gets lower and lower each night. By Memorial Day, you’ll need perfect viewing and no trees or buildings to the southwest in order to see it. How long can you follow it? All the way to May 31? Once it’s gone, the Dog Days are upon us.

Saturn remains well placed in the evening sky this month. Look for it in the south at dusk. Mercury is briefly visible at dusk for the first week in May. Mercury is bright enough to appear in twilight while most stars aren’t. Look low in the west northwest at dusk, right over the point of sunset. A compact cluster of stars called the Pleiades is nearby.

The Constellation Orion
photo credit: Wisze

Jupiter, in the south-southeast at dawn, is the brightest thing in that part of the sky unless the Moon is nearby (as it is on May 17). Venus is a dazzling morning star this month. Look east right at day break for the brightest thing there except for the Moon. Venus remains the ‘morning star’ for the rest of 2009. Mars remains close to the horizon at dawn much of the spring, and takes longer to fully emerge into the morning sky.

Look in the south at dusk for stars in the shape of a backwards question mark, with a right triangle to the left of that. These are the stars in Leo the Lion. Saturn is under the ‘right angle’ in that right triangle. The Big Dipper is highest on spring evenings. From the Big Dipper’s handle, you can arc to Arcturus. Arcturus, in the east at dusk, is the fourth brightest star we ever see at night and will be the brightest one left once Sirius sets. Continuing the curve of the Big Dipper’s handle past Arcturus, you can “speed on to Spica“, a star low in the southeast at dusk. Spica represents a stalk of wheat held by Virgo the Virgin, who is the harvest goddess.

Dazzling Orion leaves the evening sky this month; you can see him only right at dusk in early May. His belt now points right to Aldebaran, the eye of Taurus the Bull, which sets with Orion in the west. The Dog Stars Sirius and Procyon also set in the west, to Orion’s left. Sirius is the brightest star we ever see at night. Now above Orion are two stars of similar brightness less than five degrees apart. These are Castor and Pollux, marking the heads of Gemini, the Twins.

Moon Phases in May 2009:

1st Quarter May 1, 3:44 pm
May 30, 10:22 pm

Full May 8, 11:01 pm

Last Quarter May 17, 2:27 am

New May 24, 7:11 pm

Go Stargazing! May Edition

Published by James on May 1, 2008 at 4:50 pm in Astronomy

photo credit: fdecomite

Overall, the May evening sky marks a transition. Brilliant winter stars (including Sirius) are leaving the evening sky, while the stars of summer are only beginning to peek over the eastern horizon.

At dusk this month, the plane of the Milky Way roughly coincides with the horizon. We are therefore looking up out of the Milky Way plane, where there are fewer bright stars. One bright star high in the sky tonight is Arcturus, which you can find by extending the arc of the Big Dipper’s handle (‘arc to Arcturus’).

Arcturus, the fourth brightest star we see at night, will be the brightest star left once we can no longer see Sirius. The Big Dipper happens to be at its highest above the North Star at dusk this month.Mercury makes a brief appearance in the evening sky this month. Look for it in the west at dusk, right over the point of sunset.

Mars continues to fade as Earth pulls away from it. To see Mars, look high in the sky to the west at dusk to the upper left of the ‘twin’ stars of Gemini. Mars’ position among the stars changes quite noticeably on a nightly basis.

Saturn is high in the sky towards the south at dusk this month. Look to the south at dusk, almost overhead, to find stars in the shape of a backwards question mark. These form the mane of Leo, the lion. The ‘point’ under the question mark is Regulus, a star of similar brightness. Saturn is to Regulus’ left.

Jupiter is in the predawn sky this month, located in the south at dawn. It outshines everything else there unless the Moon is present.

Venus is lost in the Sun’s glare, and will remain out of sight through the end of the summer.

photo credit: 3D King

When do the Dog Days of Summer begin? Find out for yourself by noting when the last day in May is that you can still see Sirius, the Dog Star. As May opens, Sirius is easy to find; it outshines all other stars we see at night and is the brightest thing in the southwest at dusk. However, Sirius appears lower and lower to the horizon each night this month. As May comes to a close, Sirius sets deeper and deeper in twilight and finally becomes invisible by month’s end. Ancient Egyptians believed that Sirius, as the brightest star in the night sky, would reinforce the Sun’s heat if it were up during the daytime. Thus, the time of year when Sirius is not visible at night (and therefore up only in daytime) became known as the ‘Dog Days’.