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.

Shine on Harvest Moon

As this week begins, the moon is in waxing gibbous phase, on its way to being full on the morning of September 23. This full moon occurs just six hours after the fall equinox, which is at 10:13 pm the previous night.  Therefore, this is this year’s Harvest Moon.  Every year, the full moon nearest to the fall equinox is the Harvest Moon, even if the two don’t coincide as well as they do this year.  If the full moon occurs very early in September, the Harvest Moon is the full moon of early October.  To understand why the full moon nearest the fall equinox would be special to early farmers, we need to understand some celestial geometry. 

Consider two geometric planes. One is your horizon, a flat plane tangent to the Earth at your location.  The other is the plane of the Earth’s orbit around the sun.  This plane, projected against the sky, is called the ecliptic; we see the Sun shift position along this plane throughout the year as we orbit it.  The solar system as a whole is so flat that all planets orbit in nearly the same plane.  Because the moon formed from a collision between Earth and a Mars-sized object, it orbits Earth with five degrees of Earth’s orbital plane.  (Had the moon formed with the Earth, it would orbit in the plane of Earth’s equator.)  Thus the sun, all the planets, and even our moon always appear near the ecliptic in the sky.   

On winter and spring evenings, the ecliptic forms a steep angle to the horizon.  In the northern hemisphere, this is particularly true in March, as winter turns to spring.  With that steep angle, the moon’s daily displacement along the ecliptic causes it to rise just over an hour later each day.  For example, moonrise on March 29, 2010, in Houston was at 7:33pm CDT; while on the next night the Moon rose at 8:40.

On summer and autumn evenings, however, the ecliptic intersects the horizon at a shallow angle.  For observers in the northern hemisphere, this is especially true in September, when summer turns to autumn.  With that shallow angle, the time of moonrise does not change as much due to the moon’s daily displacement along the ecliptic.  If the moon is rising in the east at dusk, it will rise only about a half hour later for several days in a row.  For example, moonrise on Wednesday night, September 22, is at 6:44pm CDT.  The Moon rises on Thursday, September 23, at 7:14pm, and on Friday, September 24, at 7:45pm. 

Harvest Moon
Creative Commons License photo credit: Jay Scott Photography

This was a great help to early farmers bringing in the harvest.  On a typical evening, work would have to cease at nightfall. A full moon, though, meant that a new source of light rose right as the sun set.  Thus, harvesters could continue to work into the night by moonlight, without having to stop.  And at the Harvest Moon, the moon would rise near sunset for a few days in a row.  Harvesters had several days  of round the clock labor to bring in everything their fields had produced, leaving as little as possible to wither on the vine. 

Time and the advance of technology have diminished our connection to the cycles of nature; for many of us the coming full moon is just one of the twelve full moons this year.  However, in this month when many of us have returned to work from vacation, and have even taken time to celebrate laborers, we can reflect on how the light of our nearest neighbor helped laborers of old harvest their fields.

Go Stargazing! October Edition

Jupiter is the brightest thing in the evening sky this month, unless the Moon is out.  Face south-southeast and look for the brightest point of light there.  Remember, Jupiter outshines everything in the sky except the Sun, the Moon, and Venus, so if you’re looking in the right direction, you can’t miss it.

Conjonction Lune/Vénus
The Moon and Venus
Creative Commons License photo credit: ComputerHotline

Venus is still a dazzling morning star this month, but it’s now getting lower in the pre-dawn sky.  Look east right as day begins to break for the brightest thing unless the Moon is nearby.  Venus remains the ‘morning star’ for the rest of 2009.  Mars is now high in the east-southeast dawn. It is also brightening as the Earth approaches it. Saturn begins to emerge from behind the Sun, joining Venus in the morning sky.  Venus and Saturn are in conjunction on October 13. Look for Saturn between Venus and the horizon at dawn before that date, and slightly above Venus afterwards.  Elusive Mercury is also below Venus at dawn during the first half of the month.

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.  Sagittarius, the Archer, known for its ‘teapot’ asterism, is in the southwest (Jupiter is in Sagittarius).  Look for the enormous Summer Triangle, consisting of the stars Deneb, Vega, and Altair, high in the west.   As familiar summer patterns shift to the west, the constellations of autumn take center stage.  The Great Square of Pegasus is high in the east 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.  In fall, while the Dipper is low, Cassiopeia rides high.

Moon Phases in October 2009:

Full                                    October 4, 1:11 am
Last Quarter                  October 11, 3:56 am
New                                   October 18, 12:32 pm
1st Quarter                     October 25, 7:41 pm

Harvest Moon
Creative Commons License photo credit: Jay Scott Photography

The Full Moon of October 4 is the Full Moon nearest to the fall equinox.  Therefore, it is the Harvest Moon.  The ecliptic, which is the plane of the solar system set against the background stars, makes a very shallow angle with the horizon on late summer and early fall evenings.  Since the Moon orbits us in almost the same plane where Earth orbits the Sun, we see the Moon near the ecliptic.  When the ecliptic makes a shallow angle with the horizon, a shift in position along the ecliptic translates into less height above (or distance below) the horizon.  As a result, around the start of fall we see the Moon rise at about the same time for several days around Full Moon.  Harvesters often took advantage of this to keep working deep into the night.

November 1 is the first Sunday in November.  Therefore, Daylight Saving Time ends at 2:00 am that morning. (The time goes from 1:59:59 back to 1:00, such that the 1:00 am hour occurs twice.)  On Halloween night, remember to set your clocks back one hour and enjoy your extra hour of sleep!

Our annual Astronomy Day at the George Observatory is Saturday, October 24 this year.  From 3 p.m. to 10:30 p.m., we’ll be celebrating 400 years of modern astronomy and 20 years of the George Observatory.  Surf to www.astronomyday.org to read about all about the events going on that day.

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.