Seeing Stars with James Wooten: September 2012

Mars remains an evening object. It is low in the southwest at dusk. Saturn is now in the west-southwest at dusk for one more month. By month’s end, it sets just after twilight ends.

Jupiter emerges higher into the morning sky this month. Look for it high in the south at dawn; it outshines all stars in that direction. Venus remains high in the east at dawn, continuing a spectacular morning apparition.

The Big Dipper is to the lower left of the North Star at dusk, with its handle pointing up and to the left. From that handle, you can ‘arc to Arcturus’, which is in the west at dusk.

Sky Events | September 2012This star map shows the Houston sky at 10 p.m. CDT on Sept. 1, 9 p.m. CDT on Sept. 15, and at dusk on Sept. 30. To use the map, put the direction you’re facing at the bottom.

Antares, the brightest star of Scorpius the Scorpion, is in the southwest, with the ‘teapot’ of Sagittarius behind it. The Summer Triangle approaches the zenith (overhead point).   Pegasus, with its distinctive Great Square, has risen in the east.

Moon Phases in September 2012:
Last Quarter                  September 8, 8:15 am
New                               September 15, 9:09 pm
1st Quarter                    September 22, 2:41 pm
Full                                 September 29, 10:17 pm

At 9:47 a.m. on Saturday, September 22, the Sun is directly overhead at the equator, shifting towards the south. This, then, marks the autumnal (fall) equinox. On this date, everyone on Earth has the same amount of daytime and nighttime. Ever since the spring equinox in March, daytime has been longer than night for us in the Northern Hemisphere, while the reverse has been true in the Southern Hemisphere. After September 22, night is longer than day for us and the day is longer than the night below the equator.

The Full Moon of Saturday, September 29 is the Full Moon closest to the autumnal (fall) equinox. This, therefore, is the Harvest Moon. The angle between the ecliptic — the plane on which the Sun, Moon, and all planets appear — and the horizon is always shallowest near the fall equinox. As a result, moons near full phase at this time of year rise at almost the same time each night for a few days in a row. Farmers used this light to keep working their fields long into the night at harvest time.

On most clear Saturday nights at the George Observatory, you can hear me do live star tours on the observation deck with a green laser pointer. If you’re there, listen for my announcement.

To enjoy the stars in any weather from the comfort of the HMNS Planetarium, click here for a full schedule.

Would you like email updates on current events in the sky, at the planetarium, and at the George Observatory?  If so, send an email to astroinfo@hmns.org.

Go Stargazing! March Edition

Venus leaves the evening sky in dramatic fashion this month.  Look west-southwest right as night falls for the brightest thing there except for the Moon.  Keep watching each clear night this month; you’ll see Venus noticeably lower to the horizon each passing day.  By March 20, Venus sets as twilight ends, and by the end of the month, it is gone.  When is the last day the month you can see it?  Venus, on its faster, inner orbit, has come around to our side of the Sun and will pass us on March 27.  Astronomers call this alignment inferior conjunction

(: Smiley Face Over Perth
Creative Commons License photo credit: rich115

In addition, Venus’ orbit is highly inclined to ours.  (The planets orbit almost, but not exactly, in the same plane.)  As a result, we often see Venus pass above or below the Sun at inferior conjunction rather that truly aligning with the Sun.  This time, Venus passes ‘above’ the Sun in our sky, giving us the chance to see it as both evening and morning star!  Do you have a clear horizon, without tall trees or buildings, to the east and west?  If so, then you can try observing Venus very low in the west at sunset and very low in the east the next morning.  It’s best to try this between March 24 and 27. 

Saturn is now up all night.  On March 8, Earth passes between the Sun and Saturn, putting the Sun and Saturn on opposite sides of the Earth.  In this alignment, called opposition, a planet rises at sundown and sets at sunup; it is visible literally all night long.  Saturn is nowhere near as bright as Venus, but it is in a relatively dim star field and therefore is just as easy to see.  Face east at dusk, south at midnight, or west at dawn to see it. 

Mars and Jupiter emerge from the Sun’s glare this month.  Jupiter, in the southeast at dawn, is the brightest thing in that part of the sky unless the Moon is nearby (as it is on March 22, 23, and 24).  Mars moves faster than Jupiter and therefore seems to ‘keep pace’ with the Sun’s apparent motion.  As a result, Mars remains close to the horizon at dawn much of the spring, and takes longer to fully emerge into the morning sky. 

M42 Orion
Creative Commons License photo credit: makelessnoise

Dazzling Orion is due south at dusk.  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.  Look for a fairly bright star just to the right of Sirius and then drop your gaze straight down to the horizon. The bright star just above the horizon, possibly shining through trees, is Canopus, the second brightest star we see at night.  This star is so far south that it never rises for people north of 37 degrees north latitude (Houston is at just under 30 degrees north).  To Orion’s upper left are two stars of similar brightness less than five degrees apart.  These are Castor and Pollux, marking the heads of Gemini, the Twins.  Look in the east at dusk for stars in the shape of a backwards question mark, with a right triangle below that.  These stars are in Leo, the Lion.  Saturn rises in Leo.

Moon Phases in March 2009:

1st Quarter           March 4, 1:45 am
Full                       March 10, 9:37 pm
Last Quarter          March 18, 12:49 pm
New                      March 26, 11:07 pm

Time For... ?
Creative Commons License photo credit: bogenfreund

Sunday, March 8, is the second Sunday of March.  Therefore, we spring forward to Daylight Saving Time at 2 a.m. that morning.  (Clocks officially go from 1:59 a.m. to 3 a.m.)  Don’t forget to set your clocks one hour ahead Saturday night, March 7!

At 6:45 am on Friday, March 20, the Sun is directly overhead at the equator.  This is therefore the vernal (spring) equinox.  On this date, everyone has the same amount of daylight.  For us, day is now longer than night, and days will continue to lengthen until June.  In the Southern Hemisphere, it’s night that is getting longer.  For them, this is the autumnal equinox—the start of fall. 

A Trick or a Treat?

In less than a week, people all over the country, including right here at our museum, will be celebrating Halloween. Perhaps your workplaces and schools are already festooned with ghosts, skeletons, graveyards, and the like.  If you stop and think about it, you may wonder just how it is that we came to celebrate by trying to disguise ourselves or by trying to frighten people.  Is this a trick or a treat?

Picket fence and yellow trees
Creative Commons License photo credit: joiseyshowaa

The short answer as to why we celebrate this time of year with images of death is that we are in the middle of autumn, the season when nature itself is dying.  To fully understand why we celebrate Halloween when we do, we must fully understand the seasons.

Earth orbits the Sun with its axis pointed at the North Star, Polaris. As a result, its axis is tilted by about 23.5 degrees with respect to its orbital plane.  This tilt, combined with Earth’s revolution around the Sun, causes the seasons.  If the North Pole leans towards the Sun, the Sun is higher in our sky and we get more direct sunlight.  Also, daytime is longer than nighttime.  As the North Pole begins to tilt away fron the Sun, the Sun appears lower and lower across the sky, and daytime gets shorter and shorter.  Eventually, the slanted-in solar rays and short days bring about winter.  Very cold air masses form in the darkened Arctic and begin to move south, some of which can even reach Houston.

Keep in mind that the Earth’s axis does not tilt back and forth; it points at Polaris the whole time.  In June, the North Pole is leaning towards the Sun, but by December, the Earth’s motion has carried it to the other side of the Sun.  The North Pole, still tilting the same way, now leans away from the Sun.

A common misconception is that the Earth is closer to the Sun in summer and more distant in winter, and that is what causes our seasons.  In fact, Earth’s perihelion (closest approach to the Sun) occurs just after the new year (January 1-4), while aphelion (greatest distance from the Sun) occurs around the 4th of July.  Earth’s orbit is an ellipse, but the Earth-Sun distance does not change by enough to affect our seasons.

where are you?
Creative Commons License photo credit: shioshvili

In the cycle of seasons, there are four points of note.  At the March equinox, neither pole is tilted toward the Sun and the Sun is directly overhead at the equator.  The is the vernal (spring) equinox for us and the autumnal (fall) equinox for folks south of the equator.  At the June solstice, the North Pole is tilted as much as possible towards the Sun, and the Sun is overhead at 23.5 degrees North (the Tropic of Cancer).  This is the summer solstice for us and the winter solstice in the Southern Hemisphere.  At the September equinox, once again neither pole tilts toward the Sun, and the Sun is again overhead at the equator.  This is our fall equinox and their spring equinox.  At the December solstice, the North Pole is tilted as much as possible away from the Sun, and the Sun is overhead at 23.5 degrees South (the Tropic of Capricorn).  This is the winter solstice for us and the summer solstice below the equator. 

We generally think of these points as the beginning of spring, summer, fall, and winter, but it doesn’t have to be that way.  After all, nothing magically happens with our weather on these dates.  We could just as well consider these points the midpoints of each season.  In that case, the seasons would begin and end at points roughly halfway between the equinoxes and solstices, in early February, May, August, and November.  If the equinoxes and solstices are ‘quarter days,’ the points halfway between them become the ‘cross-quarter days.’

The ancient Celts of Europe appear to have divided their year in precisely that way.  Gauls living in what is now France used a calendar of twelve lunar months with a 13th month added every 2.5 years (similar to the Hebrew calendar today).  Their two most significant months were Gamonios (lunar month corresponding to April/May ), which began the summer half of the year, and Samonios (lunar month corresponding to October/November) which began the winter half of the year.  Julius Caesar noted that daytime followed nighttime in Celtic days.  By extension, the dark (winter) half of the Celtic year preceded the light (summer) half, making Samonios the start of their new year.

The Celts in the British Isles (Irish and Scots) also had festivals aligned with the cross-quarter days.  In early February was Imbolc (or St. Brigid’s day).  Weather predicting traditions of this day are preserved in our current Groundhog Day.  Traditional May Day celebrations are similar to those of the Celtic BeltaneLughnasadh, in early August, marked the start of the harvest. 

'' The Sentiment of Light''
Creative Commons License photo credit: jdl_deleon

The most important, though, was Samhain (pronounced ’sah win’, not ‘Sam Hane’, due to rules of Gaelic spelling), in early November.  This three-day festival marked the beginning of the winter half of the year and the start of the whole year, like Gaulish Samonios.  It was the close of the harvest opened at Lughnasagh, and the time for culling excess livestock.  At this time, the veil between the living and the world of the dead was considered thinner than usual, and people looked forward to meeting and communing with ancestors and relatives who had died.  A ‘dumb supper‘ was set aside for departed relatives.  To scare away unwanted spirits, people dressed in frightening garb.  Note that these spirits were considered unpredictable and possibly mischievous because they were not the familiar ancestors–not because they were particularly evil.  Divination was also practiced at this time, as people sought to predict whom they would marry or how many children they would have. 

Doing the math, you’ve probably figured out that Halloween is not quite halfway from the equinox (September 22) to the solstice (December 21).  But remember, the Celts used a lunar calendar.  They celebrated their festivals on a certain phase of the Moon, possibly full moon, occurring nearest the cross-quarter day.  Upon the adoption of the Julian calendar, which was not strictly lunar, the festivals were moved to the beginning of February, May, August, and November, although this meant they were no longer exactly on the cross-quarter days. 

Saint
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The Wandering Angel

In the eighth century AD, Pope Gregory III moved the church’s commemoration of the souls in heaven (All Saints’ Day) from May 13 to November 1.  Another name for All Saints’ Day is All Hallows Day.  (’Hallow’ is an older term for ’sanctify’ or ‘make holy.’  Think of ‘…hallowed be thy name’ from the Lord’s Prayer).  The next day became All Souls’ Day.  The day before All Hallows Day or All Saints’ Day is All Hallows Eve, or Halloween.  The traditions of Samhain, with its similar focus on honoring the dearly departed, were a natural fit for All Hallows Day and All Hallows Eve.

Halloween, then, is ultimately just one expression of the human need to come to terms with death as a natural occurence and to honor those who have gone before.  In the season of the fall of the leaf, with the Sun taking a slightly lower path across the sky each day, the natural world is going through its own ‘death,’ providing a perfect context for our own activities.  We can therefore think of Halloween itself as a treat, not a trick.

I wish everyone a Happy Halloween, with many more treats than tricks.

Go Stargazing! September Edition

Jupiter remains well placed for observing this month.  It outshines everything else in the night sky, unless the Moon or Venus (which is only visible in the early evening) is present.  It is therefore easy to find in the south at dusk. 

Saturn is out of sight right now, as it is directly behind the Sun from our perspective on September 3 (This alignment is called conjunction). However, Saturn emerges into the morning sky by the end of the month. Venus is beginning to re-emerge into the evening sky. Although it is low in the west southwest in the evening twilight, Venus outshines everything in the sky except the Sun and the Moon, so viewers with clear views to the west southwest should be able to find it.  Mars disappears into the Sun’s glare this month, and will remain out of sight into 2009. 

Inside The Forums at Caesar's Palace
Creative Commons License photo credit: SykoSam

The brightest star in the sky this evening is Arcturus, which you can find low in the west by extending the arc of the Big Dipper’s handle (‘arc to Arcturus’).  Arcturus, the fourth brightest star we see at night, is the brightest star left right now, since the top three are not visible in Houston during September. 

The Big Dipper happens to be to the lower left of the North Star at dusk this month; you’ll need a clear northern horizon to get a good look at it, especially later in the month.  Scorpius, the Scorpion, is in the southwest at dusk.  Sagittarius, the Archer, known for its ‘teapot’ asterism, is to Scorpius’ left (Jupiter is in Sagittarius).  Look for the enormous Summer Triangle, consisting of the stars Deneb, Vega, and Altair, overhead at dusk.  This triangle was up all night long in June and July, hence its name.  In the east, the Great Square of Pegasus has entered the sky, heralding the approaching autumn. 

Moon Phases in September 2008:

1st Quarter            September 7, 9:04 pm
Full                        September 15, 4:14 am
Last Quarter          September 22, 12:05 am
New                       September 29, 3:12 am

At 10:44 am on Monday, September 22, the Sun is directly overhead at the equator.  This marks the autumnal equinox, the official start of autumn.  On this day, everyone on the planet has the same amount of daylight.

Ever since June 21, the Sun has appeared slightly lower in the sky each day.  Also, we’ve been having a little less daylight each day since June 21.  From September 22 forward, nighttime is longer than daytime in the Northern Hemisphere.  The reverse is true in the Southern Hemisphere.  There the days have been getting longer and the Sun slightly higher in the sky since June 21.  After September 22, daytime is longer than nighttime in the Southern Hemisphere; that day is the vernal (spring) equinox for them.

Harvest Moon
Creative Commons License photo credit: maxedaperture

The Full Moon of September 15 is the closest Full Moon to the equinox.  Accordingly, it is the Harvest Moon.  The plane of our solar system in the sky, called the ecliptic, makes a very shallow angle to the horizon on September evenings.  As a result, the Moon, which we see roughly along that path, rises at almost the same time several nights in a row near the Full Moon.  Harvesters could work late into the night by moonlight with little darkness between sunset and moonrise for several days. 

Want to learn more about Astronomy?
How many planets do we really need anyway?
Why is July called July?
Want to see the longest solar eclipse of the century? China, 2009.