Go Stargazing! April Edition

Saturn, up all night long last month, can now be found in the east southeast at dusk.  We are seeing its rings a little more edge on than earlier in the year, an effect that gets even more pronounced next month.

Venus keeps getting higher in the evening sky during the month of April.  Face west at dusk and look for a point of light that outshines everything in the sky. As April opens, Venus has a companion; the elusive Mercury is to its lower right.  Normally too close to the sun to appear in our night sky, Mercury has come from behind the sun and appears far enough to its side that we can still see it just after sunset. Mercury’s greatest elongation (apparent distance from the sun) occurs on April 8.  After that date, we see Mercury return towards the sun’s glare.

Mars is very high in the evening sky, although not as bright as it was in winter.  Since Jan. 29, the 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 April, Mars remains brighter than average and thus remains easy to see.  Look high in the south 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.

Johannes Hevelius drew the Orion constellation
in Uranographia, his celestial catalogue in 1690

Now that the winter is over, the winter stars have shifted to the west.  Dazzling Orion is high in the southwest.  His belt points right to Aldebaran, the eye of Taurus, the Bull.  The Dog Stars Sirius and Procyon are to Orion’s left.  Sirius is the brightest star we ever see at night.  Gemini, the Twins, are to Orion’s upper left.  Look for two stars of equal brightness less than 5 degrees (three fingers at arms’ length) apart.  These are Castor and Pollux, marking the twins’ heads.  High in the northwest is Capella, the sixth brightest star ever seen at night.

Meanwhile, the spring stars are high in the 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 high in the northeast at dusk. If you have a clear eastern horizon, you can extend the curve of its handle to ‘arc to Arcturus’ and then ‘speed on to Spica.’  These stars are along the eastern horizon by dusk tonight.

Star Cloud Over Saskatchewan.jpg
Creative Commons License photo credit: Space Ritual

The large contrast between the bright winter stars in the west at nightfall and the dimmer spring stars in the east arises because of the shape of our Milky Way. The Galaxy is a barred spiral much thinner than it is wide across. Thus, most stars are near the plane of the galaxy.  Orion, Taurus, Gemini, and the Dogs are near the galactic plane, while Arcturus and the stars of Leo and Virgo are far above it.



Moon Phases in April 2010:

Last Quarter                  April 6, 4:37 a.m.

New Moon                      April 14, 7:30 a.m.

1st Quarter                     April 21, 1:19 p.m.

Full Moon                       April 28, 7:18 p.m.

Go Stargazing! February Edition

Jupiter!
Creative Commons License photo credit: Joshua Bury

Jupiter leaves the evening sky this month.  You can still see it during the next two weeks if you face southwest at dusk and look for the brightest point of light there. Jupiter sets by 7:30 as February opens, so you must look soon after dusk to see it.   However, Jupiter sets earlier and earlier and appears lower and lower to the horizon each February night, and soon disappears into the sun’s glare.  On Tuesday, Feb. 16, observers with a clear view of the horizon during twilight can try to see a very close conjunction of Jupiter and Venus, which is slowly moving out of the sun’s glare.  By the end of the month, Earth and Jupiter are on opposite sides of the Sun and Jupiter is therefore invisible to us.

Mars has become an evening object.  It is now already up in the east-northeast by dusk.  Mars already outshines all stars in the night sky except the very brightest (Sirius), and will continue to brighten throughout February.  On Jan. 29, Mars came to opposition as Earth passed between Mars and the sun, putting Mars in our sky all night long.  Earth now starts to pull ahead of Mars on its faster orbit.  As a result, Mars is slightly dimmer each night for the rest of 2010.  However, during February, Mars remains about as bright as the brightest stars, and thus remains easy to see.

Saturn is now high in the southwest at dawn.  Although not as bright as Mars this month, Saturn is brightening as it approaches its own opposition in March.

Joseph Nollekens (1737 - 1823) Castor and Pollux front (V&A 2007)
Castor and Pollux
Creative Commons License photo credit: ketrin1407

Dazzling Orion is high in the south, reminding us that winter is here.  His belt points up to Aldebaran, the eye of Taurus, the Bull.  The Dog Stars, Sirius and Procyon, are below Orion in the east.  Sirius is the brightest star we ever see at night.  Gemini, the Twins, are to Orion’s left as he rises (and to his upper left once they appear to the south).  Look for two stars of equal brightness less than 5 degrees (three fingers at arms’ length) apart.  These are Castor and Pollux, marking the twins’ heads.  High in the northeast is Capella, the sixth brightest star ever seen at night.  On February and March evenings, look below Sirius and a bit to its right for Canopus, the second brightest star we ever see at night. This star is in the keel (bottom) of the legendary ship Argo.  Canopus is so far south that most Americans never get to see it.  We, however, are far enough to the south that it barely rises for us, remaining low on the southern horizon.

Moon Phases in February 2010:

Last Quarter                  February 5, 5:50 pm
New Moon                      February 13, 8:52 pm
1st Quarter                     February 21, 6:42 pm 
Full Moon                       February 28, 10:37 am

The new moon of Feb. 13 is the second new moon after the winter solstice.  Accordingly, it marks the Chinese New Year, beginning the Year of the Tiger.  (Correct for the time zone difference, and you’ll see that the date is February 14 in China).

Chinese New Year - Dragon
Creative Commons License photo credit: ajagendorf25

Go Stargazing! January Edition

The full moon of Jan. 15 partially blocks the sun, causing an eclipse visible in Africa and Asia. Because it is close to apogee, the moon is too small in the sky to ever block the sun completely, and no total eclipse occurs. Instead, folks on a path stretching from Uganda and Kenya across the Indian Ocean to Sri Lanka, Burma, and China see an annular eclipse. The moon will appear to be completely inside the sun’s disk with a ring of sun around it.

Eclipsed? Not totally.
Creative Commons License photo credit: James Jordan

At about 6 p.m. on Saturday, Jan. 2, the Earth came to perihelion. This means it was as close to the sun as possible—about 147 million km away (Earth is about 152 million km from the sun in July). This is not enough of a difference to influence our weather. Earth’s 23.5 degree tilt is much more important. Our North Pole is still tilted very much away from the sun and the sun still takes a very low path across our sky. Therefore, in spite of being as close to the sun as we’ll be all year, we’re going to be quite cold in Houston this week.

The latest sunrise of the year (at Houston’s latitude) occurs on Jan. 10. Earth is now just past perihelion, and has sped up a little in its orbit since it is a little closer to the sun. As a result, sunrise, local noon, and sunset have been happening a little later each day since early December. The noon sun is ever so slightly higher at noon each day since the solstice on Dec. 21, but as of today, the sun is still very close to its solstice height. Beginning Jan. 11, the noon sun’s greater height in the sky again becomes the dominant effect (as it is for most of the year). From then until June, sunsets are getting later while sunrises are getting earlier.

Jupiter is the brightest thing in the evening sky this month, except for the moon. You can easily see it by looking to the southwest.  Jupiter appears lower and lower to the horizon each January night.  By the end of the month, Jupiter sets just after 7 p.m.  At the end of February, Earth and Jupiter will be on opposite sides of the sun and Jupiter will therefore be invisible to us.

Mars has become an evening object.  It now rises in the east about 8 p.m. and will rise by dusk at the end of the month.  Mars already outshines all stars in the night sky except the very brightest (Sirius), and will continue to brighten throughout January.  On Jan. 29, Earth will pass between Mars and the sun, putting Mars in our sky all night long (this alignment is called opposition).

Saturn
Creative Commons License photo credit: Elsie esq.

Saturn is currently high in the south at dawn.  Although not as bright as Mars this month, Saturn  is also brightening as it approaches its own opposition in March.

Venus is behind the sun (at superior conjunction) on Jan. 11 and is therefore not visible this month.

The Great Square of Pegasus is in the west at dusk.  The star in its upper left hand corner is also the head of Andromeda.  Facing north, you’ll see five stars in a distinct ‘M’ like shape—this is Cassiopeia, the Queen.  Her stars are about as bright as those in the Big Dipper, and she is directly across the North Star from that Dipper.  Since the Dipper is low and out of sight at dusk this month, Cassiopeia rides high.

Dazzling Orion is high in the southeast, reminding us that winter is here.  His belt points up to Aldebaran, the eye of Taurus, the Bull.  The Dog Stars Sirius and Procyon are below Orion in the east.  Sirius is the brightest star we ever see at night.  Gemini, the Twins, are to Orion’s left as he rises (and to his upper left once they appear to the south).  Look for two stars of equal brightness less than 5 degrees (three fingers at arms’ length) apart.  These are Castor and Pollux, marking the twins’ heads.  High in the northeast is Capella, the sixth brightest star ever seen at night.

Moon Phases in January 2010:

Last quarter moon                 January 7, 4:41 am
New moon                                January 15, 1:12 am
First quarter moon                January 23, 4:53 am 
Full moon                                 January 30, 12:18 am

True Colors in the Night Sky

Stepping outside on an April evening, you’ll notice many more bright stars in the western sky than in the east.   Those bright stars in the west are the stars of winter, still high in the west at dusk because winter officially ended about a month ago. 

Orion, the Hunter, remains well placed for observing.  As you watch him set in the west, his three-starred ‘belt’ seems parallel to the horizon, with the brighter stars Betelgeuse and Rigel above and below the belt, respectively.  Extend a line to the right from the belt.  This line points to Aldebaran, which marks the eye of Taurus, the Bull.   As the sky darkens, notice the dim V-shaped cluster right under Albebaran, marking the Bull’s face.  Go back to Orion’s belt, and this time extend a line to the left.  In this direction, Orion’s belt points at Sirius, the Dog Star, which is the brightest star we ever see at night.  Forming an almost equilateral triangle with Sirius and Betelgeuse is Procyon, the Little Dog Star. 

Standing over Orion as he sets in the west are the Twins of Gemini.  Look for two stars of roughly equal brightness and less then 5 degrees apart. (Three fingers held together at arm’s length is about five degrees).  These stars are Castor and Pollux, marking the twins’ heads.  To the right of Gemini are stars in the form of a pentagon–the stars of Auriga, the Charioteer.  The brightest star in Auriga is Capella.  

spectral-classes
  Creative Commons LicensePhoto Credit:
chipdatajeffb

April is the last full month to observe this swath of brilliant stars; they begin leaving the evening sky in May.  Not only does this set of stars include six of the twelve brightest stars at night, but it also includes at least one example of each spectral class of star.  Astronomers classify stars into seven spectral classes in order from hottest to coolest:  O, B, A, F, G, K, and M.  “Oh Be A Fine Girl/Guy, Kiss Me” is the most common phrase we use to remember the order. 

Astronomers learn what a star is made of by producing a spectrum of its light and noting which wavelengths are absorbed.  Black lines across the continuous rainbow spectrum indicate particular wavelengths absorbed, which correspond to particular gasses in the star. 

In the 1890 Draper Catalog of Stellar Spectra, Williamina Fleming divided stars into fifteen categories from A to O based on the presence of hydrogen lines in the stars’ spectra.  Only much later did astronomers figure out the more interesting relationship between a star’s spectrum and its temperature.  Annie Jump Cannon of Harvard eliminated redundant categories and merged others, leaving us with the seven modern categories in the current order.  

The hottest stars are bluish and the coolest stars are reddish, with pure white stars in between.  This may sound counter-intuitive at first.  If you think about it, however, you probably know that ‘white hot’ is hotter than ‘red hot’, and that the hottest flames are bluish. 

In the sky tonight, the three stars of Orion’s belt are O stars.   Rigel is a ‘B’ star, while Betelgeuse is in the coolest ‘M’ class.  Sirius, the brightest star in the night sky, is an ‘A’ star.  The Little Dog Star Procyon is an F star.  Capella in Auriga is a G star (as is our Sun).  The bull’s eye, Aldebaran is a K star.  At first glance, all stars look white to us, since starlight is scarcely bright enough to stimulate the color-detecting cones of our eye.  When a bright M star is close to a bright O or B star, however, you often can see the difference by contrasting the two.  I invite you then, to contrast Betelgeuse and Rigel in the sky and see if you notice a difference in color.

yellow-sunlight-2
 Creative Commons LicensePhoto Credit:
Sabrina Campagna

We often describe G stars such as our Sun as yellow.  The Sun’s rays even look yellowish when we accidentally glimpse it through, say, a canopy of leaves.  However, sunlight is in fact white.  The yellow in the Sun’s rays is an illusion created by our atmosphere. 

The Sun emits light of a wide range of wavelengths, including all colors of visible light.  This light interacts with the molecules of gas making up our atmosphere.  A molecule of gas, though, is much smaller than the wavelength of visible light.  As a result, the shorter the wavelength of the light, the more likely that the light will be absorbed or redirected after interacting with an air molecule.  This is called ‘scattering‘. 


red-sunset
Creative Commons LicensePhoto Credit: law_kevan

We often use the mnemonic device ROY G. BIV to teach the order of the colors in the rainbow: red, orange, yellow, green, blue, indigo, and violet.  (By the way, we rarely use ‘indigo’ in any other context; Isaac Newton slipped it in because he really wanted seven as opposed to six colors).  This is the order from longest wavelength (red) to shortest (violet).    Therefore, our atmosphere scatters violet and blue light much more than the other colors–this is why our sky is blue.  Although violet light is even more scattered than blue light, the sky does not look violet to us because our eyes are less sensitive to violet light than to blue light.

Now that the heavy rains have passed, we’re in for a spate of clear weather this week (of April 20).  I invite you to enjoy the clear blue sky and watch a sunset or two.  And of course, keep watching after sunset as all those bright stars appear in the west.  As you learn to appreciate them, the colors in the sky become as fascinating as those in the flowers on the ground.