Go Stargazing: April Edition

Saturn dominates April 2011 skies because yesterday, on April 3, the Earth passed between the sun and Saturn.  This alignment, called opposition, puts Saturn in the sky all night long; it rises in the east at dusk and sets in the west at dawn.

Venus’ apparition as a dazzling morning star is coming to an end.  It is getting lower and lower in the sky each morning as the angle between the solar system plane and the horizon gets shallower.  Face southeast at dawn, and you can’t miss it of you have a clear horizon.

Jupiter is directly behind the sun from our perspective on April 6 and therefore invisible all month.

Mars also remains lost in the sun’s glare all month.

A swath of brilliant winter stars continues to dominate evening skies.  These stars are now high in the west at dusk and set in late evening.  Orion, the Hunter, is in the southwest as April begins.  His two dogs, represented by Sirius and Procyon, are to his left.  Beside Orion in the west is Taurus, the Bull with Aldebaran as its eye. Gemini, the Twins, are above Orion.  The Big Dipper is to the upper right of the North Star, with its handle pointing down and to the right.  From that handle, you can ‘arc to Arcturus’ and then ‘speed on to Spica’; those stars are low in the east at dusk.  Leo, the Lion, passes almost overhead in late evening.

Below Sirius, just above the southern horizon in late twilight, is a star second only to Sirius in brightness.  This is Canopus, which marks the keel (bottom) of the legendary ship Argo Navis.  Canopus is so far south, in fact, that most Americans never see it.  From the Gulf Coast, however, Canopus does rise.  In early April, you can still see it in the evening just after dusk.

Lune
Creative Commons License photo credit: ComputerHotline

Moon Phases in April 2011:

New Moon                      April 3, 9:32 a.m.

1st Quarter                     April 11, 7:05 a.m.

Full Moon                       April 17, 9:43 p.m.

Last Quarter                  April 24, 9:46 p.m.

Sunday, April 24, is the first Sunday after the first Full Moon of spring.  Therefore, this is Easter Sunday.  This happens to be the second latest possible date for Easter.  Easter will fall on April 25, the absolute latest date, in 2038.

Go Stargazing! March Edition

Hubble Images Suggest Rogue Asteroid Smacked Jupiter
Creative Commons License photo credit:
NASA Goddard Photo and Video

Jupiter leaves the evening sky this month.  For now, you can still observe it in the west at dusk, where it sets by 8:25 on March 1.  It outshines all stars in the sky, so it’s easy to find.  Face west at dusk and look for the brightest thing there.  However, Jupiter is getting a little lower in the sky each evening.  You should be able to follow it until about the middle of the month.  By month’s end, Jupiter is lost in the sun’s glare.  On April 6, it is directly behind the sun from our perspective.

Mercury emerges from behind the sun this month and appears beside Jupiter before Jupiter fades from view.  On March 15, Mercury is about two degrees to the right of Jupiter as they both set in twilight.  As Jupiter becomes lost in the sun’s glare, Mercury remains visible low in the west at dusk for the rest of the month.

Venus remains a dazzling morning star.  It is getting lower in the sky as the angle between the solar system plane and the horizon gets shallower.  Face southeast at dawn, and you can’t miss it.

Saturn is in the southwest at dawn, getting lower in the southwest by month’s end.  This is because at the end of the month, Earth is about to pass between the sun and Saturn.  This alignment, called opposition, puts Saturn in the sky all night long; it rises in the east at dusk and sets in the west at dawn (the precise opposition date is April 3).  As a result, Saturn is also an evening object, rising in the east by 9:00 p.m. on March 1 and by dusk on the 31.

Mars, just past conjunction with the sun, remains lost in the sun’s glare all month.

A swath of brilliant winter stars continues to dominate evening skies.  These stars are now due south at dusk, but shift to the southwest later in the evening.  Orion, the Hunter, is almost due south.  His two dogs, represented by Sirius and Procyon, are to his left.  Above Orion is Taurus, the Bull with Aldebaran as its eye. Gemini, the Twins, are to Orion’s upper left.  Leo, the Lion, rises in the east.  The Big Dipper has now fully re-entered the evening sky; it is to the right of the North Star with the handle pointing down.

Below Sirius, just above the southern horizon, is a star second only to Sirius in brightness.  This is Canopus, which marks the keel (bottom) of the legendary ship Argo Navis.  Canopus is so far south, in fact, that most Americans never see it.  From the Gulf Coast, however, Canopus does rise.  March and March are the best months to see it in the evening.

Moon Phases in March 2011:

New Moon                              March 4, 2:46 p.m.

1st Quarter                             March 12, 5:45 p.m.

Full Moon                               March 19, 1:10 p.m.

Last Quarter                          March 26, 7:07 a.m.

At 6:21 p.m. CDT on Sunday, March 20, the sun is overhead at the Earth’s equator, giving everyone in the world the same amount of daylight.  This, then is the vernal equinox, the ‘official’ start of spring.  For us, days have been lengthening since December 21; by now daytime is almost as long as the night.  After March 20, daytime is longer than night for us.  For many people, however, wintry weather continues so long as arctic air masses remain in motion across North America, Europe, and Asia.

Winter time
Creative Commons License photo credit: cvanstane

People in the Southern Hemisphere had their longest days back in December; their days have since shortened to be about equal to the night.  After March 20, night is longer than day down there, so this is their autumnal (fall) equinox.

Sunday, March 13, is the second Sunday in March.  Therefore, Daylight Saving Time begins at 2 that morning (1:59:59 is followed by 3:00:00).  Don’t forget to set your clocks forward one hour on Saturday night, March 12!

What’s Your Sign? OR How The Zodiac Came To Be

On January 13, 2011, Minneapolis Community and Technical College astronomy instructor Parke Kunkle caused a stir by announcing that horoscopes are wrong because the zodiac has shifted. Not only do most people ‘belong’ to the sign immediately before the one they are traditionally assigned to, but there is a 13th ‘sign,’ Ophiuchus.

This then, is an ideal time to tell the story of what the zodiac is and how it came to be.

The Story of the Solar System
The Earth orbits the sun once a year.  This orbit defines a plane in space.  That plane, projected against the background stars, is a line in our sky which astronomers call the ecliptic.  The plane of Earth’s orbit contains the sun, so the Sun always appears on the ecliptic in our sky.

The solar system itself formed from a spinning disk of dust that flattened out as it spun.  As a result, the solar system today is so flat that all planets orbit almost (although not exactly) in the same plane.  The planet with the greatest inclination (deviation from the plane of Earth’s orbit) is Mercury, and it’s off by just seven degrees.  All planets, therefore, always appear near the ecliptic in our sky.

The best theory for the moon’s formation posits that shortly after Earth had formed, a Mars-sized body dubbed Theia crashed into Earth, throwing off debris which formed the moon.  Theia, like most everything else in the solar system, had been orbiting near Earth’s orbital plane.  As a result, our moon orbits within about five degrees of Earth’s orbital plane.  In our sky, then, the moon always appears within about five degrees of the ecliptic.

Can’t see the video? Click here.

With our sun, moon, and all planets near the same plane, only a small set of stars–those aligned with the ecliptic–can ever appear near them in the sky.

The First Astrologers
Patterns formed from these stars were therefore of great importance to observers of antiquity.   Those that we use today go back to Mesopotamia, particularly Babylonia, in about 1370 BCE.  It was about that time that Babylonians created a text called MUL.APIN, which lists all of their constellations as well as the times of year when each constellation rose with the sun. (MUL.APIN, meaning ‘The Plough,’ is the name of the first constellation listed.)

Tablet 1 of MUL.APIN also includes a list of all constellations near the path of the moon in the sky–a forerunner of our zodiac.  The 18 (or 17) star patterns on that path are:

1)  The Star Cluster                                      (The Pleiades)

2) The Bull of Heaven                                 (Taurus)

3) The Loyal Shepherd of Heaven        (Orion)

4) The Old Man                                             (Perseus)

5) The Scimitar                                             (Auriga)

6) The Great Twins                                      (Gemini)

7) The Crayfish                                             (Cancer)

8) The Lion                                                     (Leo)

9) The Seed Furrow                                    (Virgo)

10) The Scales of Heaven                         (Libra)

11) The Scorpion                                         (Scorpius)

12) Pabilsag (a Babylonian god)           (Sagittarius)

13) The Goat-Fish                                       (Capricornus)

14) The Great One                                       (Aquarius)

15) The Tails                                                 (Pisces–one of its fish)

16) The Great Swallow                              (part of Andromeda and Pisces–the other fish)

17) Anutitum (a goddess)                        (part of Andromeda)

18) The Hired Man                                       (Aries)

There is some disagreement as to whether patterns 15 and 16 represent one or two constellations, hence the uncertainty as to whether the list has 17 or 18 members.

The Ancient Greeks, by about the sixth century BCE, had modified that list and produced a zodiac more like the one we use today.  They did so by leaving out stars in Orion, Perseus, Auriga, and Andromeda, which are a bit off the ecliptic itself (although the moon, which deviates by up to 5 degrees, can pass through them).

The Greeks also treated the Pleiades and Taurus as one constellation.  Virgo, the Virgin, is nearly always depicted with a stalk of wheat in her left hand, revealing her association with agriculture, like the furrow.  Babylonians had depicted Pabilsag as a composite creature armed with a bow and arrow; the Greek centaur shooting an arrow which we call Sagittarius is a simplification of this.

Babylonians often associated the Hired Man with Dumuzi, a legendary shepherd.  This may have influenced the change from ‘Hired Man’ into Aries, the Ram. The Ancient Greeks made the Babylonian ‘Scales’ constellation into the claws of Scorpius, the Scorpion, but the Romans reintroduced the Scales, putting the zodiac in its current form.


Of all the objects to appear only in the zodiac, by far the most important was the sun.  By noting which zodiacal constellations rose just before the sun and set just after the sun, early observers could use the changing position of the Sun against the background stars as a guide to the seasons.

Early lists of Babylonian patterns listed in MUL.APIN, possibly reflecting incipient stages in its formation, typically include the Bull, possibly indicating plowing season, the Lion, perhaps a symbol of the oppressive summer sun, since the sun rose with these stars in summer, the Scorpion, an emblem of death representing autumn, and the Water Bearer, representing the rains of winter.  Also often appearing on these early partial lists are the farrow and the goat/fish.  The former could represent the harvest season which follows the oppressive heat represented by the lion.  The latter is likely to represent Ea, Babylonian god of the waters, as the goat and the fish are animals associated with him.

When astrologers began using the positions of the planets, sun, and moon to describe people’s personalities, they focused on the sun.  The zodiac sign behind the sun (and thus not visible at night) on someone’s birthday was supposed to be most influential in determining that person’s character and destiny.  Although no evidence has established any connection between the apparent position of the sun and personality, belief in ‘sun signs’ continues to this day.

However, the stars’ positions in the 21st century are not the same as in antiquity.
As Earth orbits the sun, it wobbles.  After all, Earth could spin without wobbling only if no other forces whatsoever were acting on it, which is not the case.  However, Earth’s wobble is not as chaotic as it might be because we have a Moon relatively large for a planet as small as Earth.

With the Moon as a ‘counterweight,’ the Earth’s wobble becomes a more orderly precession in which the Earth’s axis describes an apparent circle on the sky once every 26,000 years.  This same precession causes the position of the sun on a given date to shift slightly–by about one degree every 72 years.  Since millennia have passed since Babylonians created the zodiac (about 1370 BCE) and since Romans finalized it (about 1 CE), the sun no longer aligns with the same patterns during the same seasons.

This brings us back to Kunkle’s announcement a few weeks ago.
It turns out that the dates traditionally associated with the ‘sun Signs’ are valid only for about the year 1 CE.  In general, the constellation actually behind the sun on your birthday is the one immediately before your traditional ‘sign.’  For example, astrologers would call me a Gemini, but the sun was in fact aligned with the stars of Taurus, the Bull, on my birthday.  You can compare the traditional dates and the actual constellations here. (The table is towards the bottom of the page).

Under the Milky Way
Creative Commons License photo credit: jurvetson

This is not a new discovery.
The ancient Greek astronomer Hipparchus noted that the stars Spica and Regulus were in slightly different positions in his time than on his predecessors’ star maps.  From this, he was able to deduce in the second century BCE that precession was occurring.  Astronomers have thus known of this effect for over two millennia.

So have astrologers, who maintain that they can still cast horoscopes because their ‘signs’ refer to fixed sectors of the sky and not to constellations.  As it happens, the traditional dates do roughly reflect when the sun would have aligned with the constellations about 2000 years ago.  Astrologers fail to explain why the constellations’ positions of 2,000 years ago might be magically relevant, however.

In 1930, astronomer Eugène Delporte helped fix the official constellation boundaries used by the International Astronomical Union.  These boundaries place a sizable chunk of the ecliptic in the constellation of Ophiuchus, a legendary healer who holds a large snake (Serpens) and stands on top of Scorpius, the Scorpion.  His stars are not on the Path of the Moon in MUL.APIN, although stars at some distance from the ecliptic, such as those in Orion or Perseus, are.  However, Ptolemy included this pattern in his list of 48 constellation in the Almagest.  Traditional skymaps of antiquity usually show the ecliptic passing through the Scorpion’s upper claw and legs, with Ophiuchus superimposed on Scorpius and standing on the ecliptic as if balancing on a high wire.  This is what may have influenced Delporte to assign most of that section of the ecliptic to Ophiuchus.

The idea of Ophiuchus as the ’13th sign’ is not new either.
Astronomers have been using Ophiuchus to point out the arbitrariness of astrology for at least 40 years.  Ophiuchus has been standing on the ecliptic for millennia, his right foot much closer to the planets than Scorpius’ stinger.  If the band of the ecliptic has powers over us, why doesn’t Ophiuchus partake of that power?  Several other constellations come near (but are not on) the ecliptic, including Cetus the Whale and Sextans, the Sextant.  The Moon and planets, which deviate by a few degrees from the ecliptic, can appear in them.  Should we factor them in as well?

Astrology vs. Astronomy
The ‘new’ dates for the zodiac signs and the ’13th sign’ Ophiuchus serve to underscore the difference between astrology and astronomy.

Astronomy is a science.  Astronomers study real planets, stars, and galaxies to learn about the real universe around us.

Astrology is myth-making.  The real positions of the stars do not matter to astrologers because astrology has more to do with mankind’s psychological needs. These include the need to see patterns and impose meaning and order onto the world and the need to feel in control of our surroundings.  Astrology thus offers the comfort of feeling that apparently random events might be predictable and controllable.

But since the astrologer’s predictions are ‘..not in our stars, but in ourselves,” as Shakespeare might say, astrology offers none of the wonder and excitement that comes from seeing the celestial bodies as they actually are, apart from our needs and desires for influence.  For that, I recommend astronomy. 

Go Stargazing! January Edition

Jupiter, now in the west at dusk, dominates this month’s evening skies.  It outshines all stars in the sky, so it’s easy to find.  Face west southwest at dusk and look for the brightest thing there.

Venus remains a dazzling morning star.  Face southeast at dawn and you can’t miss it.

Saturn is in the south southwest at dawn, above the much brighter Venus.

Mars is still lost in the sun’s glare; it will remain invisible to us all winter as Earth passes around the far side of the sun from it.

The Great Square of Pegasus sets in the west, while brilliant winter stars shine in the south.  Orion, the Hunter, is almost due south.  His two dogs, represented by Sirius and Procyon, are to his left.  Above Orion is Taurus, the Bull with Aldebaran as its eye. Gemini, the Twins, are to Orion’s upper left.

Moon Phases in January 2011:

New Moon                              January 4, 3:03 a.m

1st Quarter                             January 12, 5:32 a.m

Full Moon                               January 19, 3:22 a.m.

Last Quarter                          January 26, 6:58 p.m.

The new moon of Tuesday, January 4, partially blocks the sun, causing a partial solar eclipse.  This event occurs during our nighttime, however; the eclipse is visible only in Europe, western Asia, and northern Africa.

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

At about 1 p.m. on Monday, January 3, the Earth is as close to the sun as it will get all year. In other words, Earth is at perihelion. Earth’s orbit is not a perfect circle but an ellipse, so its distance from the sun varies between about 147 million kilometers in January and 152 million kilometers in July. This variation is too small to affect our seasons; the effect of Earth’s 23.5 degree title on its axis dominates it. That’s why it’s colder now than in July. The actual moment of perihelion varies each year between late on January 1 and early on January 5.

At Houston’s latitude, the latest sunrise of the year occurs Friday, January 10.  Of course, days have been lengthening since the solstice, which makes sunset later and sunrise earlier.  However, Earth is still going a little faster than average on its orbit, since it is just past perihelion (its closest approach to the sun).  This causes sunrise, local noon, and sunset to occur slightly later each day.  Until mid-January, we are still close enough to perihelion that the second effect actually predominates.  As a result, sunset gets a little later during early January even while the days are getting longer.