Category: Astronomy

2019, September 23: Equinox: Equal Night or Equal Day and Night


(Image Credit: NASA)

The sun reaches the Autumnal Equinox (as considered from the Northern Hemisphere) on September 23, 2019 at 2:50 a.m.  CDT. 

Since our planet is tilted about 23.5 degrees, the sun appears higher in the sky during the summer season and lower during the winter.  The sun’s rising and setting positions change throughout the seasons as well.   During late March and late September, the sun rises at the east cardinal point and sets at the west cardinal point.  Its arc across the sky is halfway between the extremes of Summer and Winter.

The sun’s light shines most directly at the equator.  Those folks living there see the sun pass overhead.  The days are nearly shadowless. because of the sun’s path as seen from the equatorial region.

The day is called “Equinox,” sometimes translated as “equal night.”  The sun is in the sky for approximately 12 hours from the equator to higher latitudes.  Sometimes we tell school children that we have “12 hours of daylight and 12 hours of night.”

Consider this: During a 24-hour period, there are three cycles of light.  Daytime, the period when the sun is above the horizon and brightly illuminates the ground.  Dark, the period when the sky is completely without sunlight and twilight and it is as dark as it gets naturally.  Twilight, the period be when the sun is below the horizon, but it illuminates the sky.  As the sun sets, the sky is bright and it’s easy to see terrestrial features.  This is known as Civil Twilight.  During this twilight phase, streetlights turn on.  If you know where to look, Venus and Jupiter can be seen

The sky continues to darken.  From a location with a good horizon, Nautical twilight occurs when the horizon is barely visible.  This is easier to at sea and so the name.  Brighter planets and stars become visible.  This occurs about 60 minutes after sunset.  Then the sky continues to darken until the last light shines from the western horizon.  This is Astronomical Twilight.  This takes about 90 minutes.  Until the process reverses in the morning, the sky is dark, as dark as it gets naturally.

So when I look at this for my latitude, on September 23, daylight is 12 hours, 8 minutes long; twilight, 3 hours, 7 minutes; and darkness, 8 hours, 45 minutes.

At my latitude “Equal Darkness,” when the daylight hours are equal to the dark hours, is October 30. Daylight and darkness are equal at 10 hours, 26 minutes. Twilight takes up the balance that is divided between the pre-sunrise hours and post-sunset hours.



2019, September 24: The Moon and Beehive Cluster

September 24: As the sky begins to brighten in the morning, about 90 minutes before sunrise, look east for the waning cresent moon.  It is 25.0 days old (past the New phase) and 25% illuminated.  Castor and Pollux, the Gemini Twins, are above the moon and Procyon is to the moon’s right.  Regulus is far below the moon.

With a binocular look below the moon for a small smattering of stars.  They look like diamonds on the velvet of the darker sky.  This is the Beehive Cluster.  The cluster is about 500 light years away and it contains about 350 stars, a few dozen of which can been seen with a binocular.

If you have sharp eyesight, look for the cluster without a binocular. To the unaided eye, the cluster resembles a cotton ball.

The cluster is also known as the Praesepe (Manger).  More formally it is known by its catalog numbers M44 or NGC 2632.

Tomorrow morning, September 25, the moon is below the cluster and still above Regulus.  Take a look!

(As an aside, look at the moon through the binocular.  On the night portion, notice that it is slightly illuminated.  This is from sunlight reflecting from the nearly full Earth, as seen from the moon, gently illuminating the night portion of the moon — Earthshine.)

2019, August 29: Sirius in Eastern Morning Sky

Now appearing in a darker sky, Sirius joins the bright stars in the eastern sky.  The sky’s brightest star is now past its heliacal rising.  The Winter Triangle – Sirius, Betelgeuse, and Procyon – is easy to locate.  Betelgeuse is visible with Rigel, stars in Orion.  Farther north, the stars of Gemini – Castor and Pollux are visible.

2019, August 12-13: Perseid Meteor Shower Peak Dimmed by Bright Moon

Meteor Shower (NASA Photo)

Annual meteor showers occur when our planet passes through the dusty debris that are spread broadly along a comet’s celestial path.  Each year during mid-August, Earth passes through the track of Comet Swift-Tuttle.  Tiny bits of dusty rock smash into our planet’s atmosphere and are vaporized in a quick streak of light — a meteor or shooting star.  During this time, the meteors can be seen anywhere in the sky, but they seem to emerge from the region of the constellation Perseus in the northeastern sky during the early evening.  As Earth rotates during the night, the point of emergence (radiant) moves higher in the sky.  The best viewing occurs after midnight when the radiant is highest in the sky before morning twilight begins,

It is important to note that the rates that are published in the popular press include dim meteors and those that dazzle our eyes and temporarily leave a streaked glow in the atmosphere.  It is impossible for any one observer to see all the projected meteor counts.  Today many live in cities or larger metropolitan where the glow of bright streetlights wash out the dimmer nighttime stars and dimmer meteors during this shower.

So, for observers in metropolitan areas should half the projected rates (60 meteors per hour).  So we’re down to 30 meteors per hour for most observers in metropolitan areas.  To see the entire sky, the observer needs an all sky camera to catch all the meteors or four friends who look above the cardinal directions while the single observer looks overhead.  So, a lone observer can see 5-6 meteors per hour in a metropolitan area or 10-12 per hour in a dark location. So be patient, especially this year with a bright moon in the sky during the morning.

This year, a gibbous moon adds bright light, so the observable rate is reduced further to perhaps 2-3 in town, 4-6 away from lights.

Here are notes for the mornings of August 11-13 that include some planet observations.  Times are in Central Daylight Time for observers near Chicago Illinois.

  • August 11: The Perseid meteor shower is near its peak. The moon sets a few minutes after 2 a.m. CDT and morning twilight begins about 2 hours later.
  • August 12: Jupiter is now setting before 1 a.m. CDT. For the Perseids, moonset occurs near 3 a.m. CDT and morning twilight begins an hour later. Bright Mercury is 10° up in the east-northeast, 30 minutes before sunrise.
  • August 13: For the peak morning of the Perseids, the moon sets about 15 minutes before morning twilight begins. About 30 minutes before sunrise, Mercury, a little brighter than yesterday morning, is nearly 10° up in the east-northeast.

2019, August 8 – 12: Moon and Evening Planets Jupiter and Saturn

As the moon reenters the evening sky it passes Jupiter and Saturn.  The chart above shows the southern sky about 9 p.m. CDT in the Chicago area, about 1 hour after sunset. (The moon’s size is exaggerated on the chart.)  Here are highlights:

  • August 8:  In the evening, one hour after sunset, the moon (8.0 days past the New phase, 64% illuminated), over 28° up in the south-southwest, is 1.9° to the upper right of Graffias, the second brightest star in Scorpius. At the same time the moon is over 10° to the upper right of Antares. the brightest star in the Scorpion, and nearly 12° to the right of Jupiter.
  • August 9:One hour after sunset, the moon (9.0d, 74%) – 27° up in the south – is 2° to the upper left of Jupiter.  At the same time the moon is nearly 9° to the upper left of Antares.
  • August 10: One hour after sunset, the moon (10.0d, 82%), 25° up in the south, is nearly midway between Jupiter and Saturn. The gaps: Moon – Jupiter, 14°; Moon – Saturn, 16°.  Jupiter is nearly 26° up in the south.  Saturn is nearly 22° up in the south-southeast.
  • August 11: One hour after sunset, the moon (11.0d, 89%) is 22° up in the south-southeast, 3.9° to the right of Saturn.
  • August 12: One hour after sunset, the waxing gibbous moon (12.0d, 95%), 18° up in the southeast, is nearly 9° to the lower left of Saturn.

2019, November 28: A Close Venus – Moon Conjunction

As Venus enters the evening sky and appears with Jupiter and Saturn, the moon passes Venus on the evening of November 28.  The moon appears 1.9° to the upper left of the Venus.

The chart above shows the scene about 45 minutes after sunset, looking southwest.  Locate an observing location free from obstructions, such as trees, houses, and buildings.  Venus is only 7° up in the southwest.

Use a binocular to note the thin crescent and that the night portion is gently illuminated by sunlight reflecting from Earth.  The moon is only 6% illuminated and 2.3 days past its New phase.

Venus is the brightest “star” in that part of the sky.  Jupiter is slightly dimmer and to the lower right of Venus.  Saturn is to the upper left of Venus.

Venus and the moon appear in the viewfinder of a camera with a 300 mm focal length lens.  A longer exposure reveals Earthshine on the moon.

2019, December 10: A Venus-Saturn Conjunction

Venus has a conjunction with Saturn on December 10, 2019.  Venus is beginning an appearance in the evening sky. The brilliant planet passed Jupiter on November 24, and began to approach Saturn.   This is the second conjunction with Saturn during the Ringed Wonder’s current apparition (appearance).  Watch Venus close the gap on Saturn and pass it on December 10.

The passing of these two planets is a slow moving show that occurs over several nights.  First, find a clear horizon in the southwest, free from trees, houses, buildings, and other possible obstructions.

In the charts that follow, several of them are displayed for a time interval after sunset.  Use local sources for the time of your sunset.  The U.S. Naval Observatory has an online calculator that displays a year of sunrises and sunsets.  Enter your state and city into Form A on the website. For readers outside the U.S., enter your longitude and latitude in Form B for your yearly table.  Click here.

2019, December 2: Venus is about 10 degrees to the lower right of Saturn.

On December 2, 45 minutes after sunset, shows Venus about 9° up in the southwest. It is about 10° to the lower right of Saturn.  On the next evening, December 3, the three evening planets – Venus, Jupiter, and Saturn – are nearly equidistant tonight, but they are not along the same arc in the sky: Venus – Saturn, 8.6°; Venus – Jupiter, 9.7°.

2019, December 5, Venus passes the star Sigma Sagittarii. Venus is about midway between Jupiter and Saturn.

Venus continues to move eastward compared to the starry background toward Saturn.  On December 5,  Venus passes 1.9° to the upper right of Sigma Sagittarii.  Venus continues to close the gap on Saturn. Venus – Saturn separations until the conjunction: Dec 7, 4.3°, Dec 8, 3.3°; Dec 9, 2.4°.

Venus passes Saturn on December 10. At mid-twilight, Venus, over 11° up in the southwest, is 1.8° to the lower left of Saturn. Venus – Saturn gaps after the conjunction: Dec 11, 1.9°; Dec 12, 2.5°; Dec 13, 3.4°, Venus is to the upper left of Saturn; Dec 14, 4.4°, Dec 15, 5.4°.

Venus continues eastward against the starry background, moving farther away from Saturn. on December 19, one hour after sunset, Venus, 12° up in the southwest, is nearly 10° to the upper left of Saturn. Venus moves into Capricornus.

Venus-Saturn Conjunctions, 2021-2025

Conjunctions of Venus and Saturn are not rare, but they are infrequent enough for us to take notice.  The table below describes upcoming conjunctions of the two planets.

Date Location Separation Description
February 6, 2021 Southeast before sunrise. 0.5° This is a very difficult conjunction to see.  Venus is only 2° up 10 minutes before sunrise.
March 29, 2022 East-southeast before sunrise. 2.1° About an hour before sunrise, the pair is easy to see. Venus is to the upper left of Saturn.  Mars is nearby, 4.4° to the upper right of Saturn.  On the morning before the conjunction, the waning crescent moon joins the scene.
January 22, 2023 West-southwest after sunset. 0.3° The pair is 8° up one hour after sunset.  Venus is left of Saturn.  The waxing crescent moon is about 8° to the upper left of Venus on the evening before the conjunction
March 21, 2024 East before sunrise. 0.6° This is another difficult conjunction to view.  The pair is less than 5° up 10 minutes before sunrise.  Venus is to the upper right of Saturn.
January 20, 2025 Southwest after sunset. 2.2° This is an easily viewed conjunction.  Venus is to the upper left of Saturn.  The pair is over 20° up in the southwest 2 hours after sunset.