09/24/2021 – Ephemeris – Capricornus, home this season to Jupiter and Saturn
This is Bob Moler with Ephemeris for Friday, September 24th. Today the Sun will be up for 12 hours and 3 minutes, setting at 7:35, and it will rise tomorrow at 7:33. The Moon, halfway from full to last quarter, will rise at 9:26 this evening.
Nearly 2000 years ago the southernmost of the constellations of the zodiac was Capricornus the water goat. That’s why the latitude on the Earth where the Sun is overhead on the winter solstice is called the Tropic of Capricorn. Not anymore, Sagittarius, one constellation west, has that honor today*. Capricornus is large, but made up of dim stars. To me, it looks like a 45 degree isosceles triangle, long side up, but which all the sides are sagging. The constellation is found low in the south at 10 to 11 p.m. The image that is supposed to be represented by the stars is that of a goat whose hind quarters are replaced by a fish’s tail, not a mermaid but a mer-goat. This autumn, Jupiter is at the left end of Capricornus, with Saturn on the right.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.
The reason for the shift is lunisolar precession, which I talked about yesterday.
Addendum
Capricornus w/Jupiter & Saturn finder animation for 10 pm, September 24, 2021. Created using Stellarium and GIMP.
09/23/2021 – Ephemeris – The Earth’s axial tilt gives us our seasons
This is Bob Moler with Ephemeris for Thursday, September 23rd. Today the Sun will be up for 12 hours and 6 minutes, setting at 7:37, and it will rise tomorrow at 7:32. The Moon, 3 days past full, will rise at 9:03 this evening.
The Earth has an axial tilt* of about 23 and a half degrees, which gives us our seasons. Because the Earth rotates on its axis, it has a slight equatorial bulge. Earth’s polar diameter is 7,900 miles (12,714 kilometers) while its equatorial diameter is 7,926 miles (12,756 kilometers), a difference of 26 miles (42 kilometers). The gravitational tug on that equatorial bulge by the Moon and Sun actually keeps the tilt stable, but does cause the Earth’s axis to precess like a top slowing down. It’s why Polaris will no longer be our North Pole star in centuries to come, just as it wasn’t in centuries past. It’s also why the constellations of the zodiac no longer align with the astrological signs of Ptolemy’s zodiac of the second century AD.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.
* Astronomers call axial tilt “obliquity” or “obliquity of the ecliptic”.
Addendum
The Moon and Sun’s gravitational force act on the Earth’s equatorial bulge, attempting to cause the Earth to straighten up and fly right. Because the Earth is spinning, it acts like a gyroscope and the torque to straighten it up causes it to be applied 90 degrees away in the direction of the rotation causing the procession. Image credit: Open Course: Astronomy.
Precession of a spinning top: the spin axis traces the surface of a cone. The axis, in the case of the Earth, traces a circle of radius 23.5 degrees on the sky. Credit NASA.
The 25,700-year cycle of precession traced on the sky as seen from near the Earth. The current North Pole star is Polaris (top). In about 8,000 years it will be the bright star Deneb (left), and in about 12,000 years, Vega (left center). The Earth’s rotation is not depicted to scale – in this span of time, it would actually rotate over 9 million times. Credit image: Tfr000, caption: Wikipedia.
09/22/2021 – Ephemeris – Autumn starts this afternoon and a look at the naked-eye planets
This is Bob Moler with Ephemeris for Wednesday, September 22nd. Today the Sun will be up for 12 hours and 9 minutes, setting at 7:39, and it will rise tomorrow at 7:31. The Moon, 2 days past full, will rise at 8:43 this evening.
Here on Earth, the season of fall will begin at 3:21 this afternoon. Let’s search for rest of the naked-eye planets for this week. Venus should be visible in the southwestern evening twilight by 8 o’clock tonight. It will set at 9:08 pm. By 8:15 pm, Jupiter and Saturn will be seen low in the southeastern sky. The brighter Jupiter will be easy to spot at that hour. Saturn will be dimmer, but a bit higher and to its right. They will be visible into the morning hours, with Saturn setting first at 2:55 am, with Jupiter following at 4:19. Saturn’s rings can be seen in a spotting scope of about 20 power magnification. Though at that power the rings won’t appear separated from the planet, so Saturn will look like an elliptical disk.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.
Addendum
Venus in twilight at 8 pm, or about 20 minutes after sunset, tonight, September 22, 2021. An animation showing its altitude at that time of 10 degrees (about the width of your fist held at arm’s length). Created using Stellarium and GIMP.
Waning gibbous Moon, about 15 minutes after rising as it might be visible in binoculars or small telescope. Created using Stellarium.
Telescopic views of the bright planets (north up) as they would be seen in a small telescope, with the same magnification, this evening. Venus at 8 pm, Jupiter and Saturn at 10 pm, September 22, 2021. Apparent diameters: Venus, 17.61″, and 65.3% illuminated; Saturn 17.86″, its rings 41.61″; Jupiter, 47.20″. The ” symbol means seconds of arc (1/3600th of a degree.) Click on the image to enlarge it. Created using Cartes du Ciel (Sky Charts).
Planets and the Moon at sunset and sunrise of a single night, starting with sunset on the right on September 22, 2021. The night ends on the left with sunrise on the 23rd. Click on the image to enlarge it. Created using my LookingUp program.
09/21/2021 – Ephemeris – Autumn starts tomorrow
This is Bob Moler with Ephemeris for Tuesday, September 21st. Today the Sun will be up for 12 hours and 12 minutes, setting at 7:41, and it will rise tomorrow at 7:30. The Moon, 1 day past full, will rise at 8:23 this evening.
Enjoy the last full day of summer. Summer will last until 3:21 pm (19:21 UTC) tomorrow, when the center of the Sun will cross the celestial equator, an imaginary line above the Earth’s equator, heading southward. At that instant, autumn will begin. Shortly after, the Sun will be up less than half the day. The day and the point in the sky that the Sun crosses is called the autumnal equinox. The word equinox means equal night, implying that day and light are of equal length. Geometrically that’s true, but the Earth’s atmosphere and the definition of sunrise and sunset, prolong daylight by a few minutes. The amount of heat we are getting and will get from the Sun cannot sustain our current temperatures, and it will get a lot colder on average before it gets warmer again.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.
Addendum
That is not a pumpkin on the head of the motorcyclist. That’s the Sun rising as I’m traveling east on South Airport Road south of Traverse City, MI on the autumnal equinox. This is the east-west section of the road. The Sun is rising over the hills some 6 miles to the east. When the Sun is on the celestial equator, it rises due east and sets due west. Credit: Bob Moler.
The Sun’s path through the sky on an equinox day from the Traverse City/Interlochen area in Michigan. The Sun is plotted every 15 minutes. This is a stereographic projection which compresses the image near the zenith and enlarges the image towards the horizon. Created using my LookingUp program.
Earth 3 days before autumnal equinox, 1:17 pm EDT, September 19, 2021. The mitten shape of Michigan was in the clear that day and is visible above center of the image. The image taken by the Earth Polychromatic Imaging Camera (EPIC) on the DSCOVR satellite in halo orbit around the Earth-Sun L1 Lagrange point, about a million miles sunward of the Earth. At the time, DSCOVR was 892,682 miles (1,436,797 kilometers) from Earth.
09/20/2021 – Ephemeris – The Harvest Moon rises tonight
This is Ephemeris for Monday, September 20th. Today the Sun will be up for 12 hours and 15 minutes, setting at 7:43, and it will rise tomorrow at 7:29. The Moon, at full today, will rise at 8:03 this evening.
Tonight’s full moon is the Harvest Moon. It is the most famous of the named full moons, and was very useful in the days before electric lights. The reason is that the Moon, around the time it is full now, doesn’t advance its rising time very much from night to night, effectively extending the light of twilight to allow more time to gather in crops. This is because the Moon is moving north as well as eastward. The farther north it is, the longer it stays up and retards the advance in rise times. On average, the Moon rises 50 minutes later each night. This week, the interval is down near 20 minutes advance in moonrise times per day, extending twilight and the time each day to harvest the crops for a few more days.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hr). They may be different for your location.
Addendum
The autumn vs spring sunset ecliptic. I’m using the autumnal equinox 2021, with the tip of the spout of the Sagittarius Teapot at due south, and vernal equinox 2022, with the red star Betelgeuse in Orion at due south as examples. Click on the image to enlarge it. Created using Stellarium and GIMP.
The ecliptic for the autumnal equinox runs low in the south, a preview of the Sun’s apparent travel for the next six months of fall and winter. Besides the planets, the Moon at sticks close to that line, as do the planets. The full moon rising in the east as the Sun sets does so at a shallow angle, so for a week or so around the full moon, its advance in rise times can be as little as 20 minutes per night. In spring, it can be much longer than an hour.
09/17/2021 – Ephemeris – The Moon has an atmosphere of sorts; Plus, I capture Inspiration 4 passing overhead
This is Ephemeris for Friday, September 17th. Today the Sun will be up for 12 hours and 24 minutes, setting at 7:49, and it will rise tomorrow at 7:25. The Moon, 3 days before full, will set at 4:24 tomorrow morning.
Jupiter, tonight, will be above the Moon. Everyone knows that the Moon is airless, that is has no atmosphere. That is not strictly true. Apollo astronauts saw glows and crepuscular rays in the direction of the Sun before its rising an after it set as they orbited the Moon. In 2013 NASA sent an orbiter spacecraft named LADEE to the Moon to investigate. LADEE stands for Lunar Atmosphere and Dust Environment Explorer. It orbited low over the Moon’s equator. The atmosphere of the Moon consists of some gasses like sodium, neon, argon and helium plus dust electrostatically charged by the Sun’s x-ray and ultraviolet emissions and sent aloft, up to many kilometers. The dust will fall back down, but the gasses will be dissipated by the solar wind. It’s very tenuous.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT – 4 hr). They may be different for your location.
Addenda
Jupiter, Saturn and the Moon tonight at 9 pm tonight, September 17, 2021. Jupiter will be moving to be directly below Jupiter by one in the morning. Created using Stellarium.
Apollo 17 twilight ray sketches. Credit: NASA/Apollo 17 Astronauts.
Artist’s depiction of LADEE in orbit of the Moon. Courtesy NASA.
Inspiration 4 spacecraft caught flying over Northern Michigan
The thin vertical streak on the left side of the image is the Inspiration 4 spacecraft against the stars of Hercules in a 15 second exposure with a Canon Rebel T5 camera, f/3.5, ISO 6400, 18 mm fl. The famous Keystone of Hercules asterism is to the upper right of center. It’s a bit out of focus due to my problem seeing the screen at a high angle. (Getting old). It was quite dim, so I actually couldn’t see it in the moonlight. Credit: mine.
I was guided to the time and place in the sky by heavens-above.com.
09/16/2021 – Ephemeris – How safe is the Space Launch System (SLS) compared to the Space Shuttle?
This is Ephemeris for Thursday, September 16th. Today the Sun will be up for 12 hours and 27 minutes, setting at 7:51, and it will rise tomorrow at 7:24. The Moon, 3 days past first quarter, will set at 3:10 tomorrow morning.
Tonight, the planet Saturn will appear above the bright gibbous Moon. It might be a bit hard to spot.
The big NASA moon rocket, the Space Launch System or SLS, borrowed a lot of features from the Space Shuttle. Is it any safer? The answer is yes. The Space Shuttle had no abort system for a catastrophic failure, as we saw with the Challenger accident in 1986. However, even though the SLS uses the same type solid rocket boosters that failed Challenger, the Crew in the Orion capsule is on top of the rocket with an escape system to rocket them away from an exploding core stage. With the Orion capsule, the escape tower will be jettisoned at some point before reaching orbit, possibly making escape impossible in the latter stage of powered flight. Both SpaceX’s Dragon and Boeing’s Starliner have escape rockets built in for the entire flight.
Addendum
Shuttle-SLS comparisons. Note that this image is 10 years old. The center core stage will, like the external tank on the shuttle, remain unpainted. So far, the cargo version of the is still in the design stage. The Multipurpose Crew Vehicle is now called the Orion Spacecraft. Credit: How Stuff Works.
09/15/2021 – Ephemeris – Let’s search for the naked-eye planets for this week
This is Ephemeris for Wednesday, September 15th. Today the Sun will be up for 12 hours and 30 minutes, setting at 7:52, and it will rise tomorrow at 7:23. The Moon, 2 days past first quarter, will set at 1:59 tomorrow morning.
Let’s search for the naked-eye planets for this week. Venus should be visible in the western evening twilight before 8:15 tonight. It will set at 9:19 pm. By 8:30 pm, Jupiter and Saturn will be seen low in the southeastern sky. The brighter Jupiter will be easy to spot at that hour. Saturn will be dimmer, but a bit higher and to its right. Both these planets will be to the left of the bright gibbous Moon. They will be visible for most of the night, with Saturn setting first at 3:24 am, with Jupiter setting at 4:50 tomorrow morning. Saturn’s rings can be seen in a spotting scope of about 20 power magnification. Though at that power, the rings won’t appear separated from the planet, so Saturn will look like an elliptical spot. Most of Jupiter’s 4 brightest moons can even be spotted in binoculars.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT – 4 hr). They may be different for your location.
Addendum
Venus in evening twilight at 8:15 tonight, about 20 minutes after sunset, September 15, 2021. Created using Stellarium.
Jupiter Saturn and the Moon at 8:30 tonight, about 40 minutes after sunset, September 15, 2021. Created using Stellarium.
Low magnification view of the Moon tonight. The large crater Clavius is seen at the south (bottom) of the Moon. The crater Copernicus is seen near the left edge of the Moon, the sunrise terminator. Created using Stellarium.
Telescopic view of the bright planets (north up) as they would be seen in a small telescope, with the same magnification, this evening. Venus at 8:30 pm, September 15, 2021. Apparent diameters: Venus, 16.70″ 68% illuminated; Saturn 18.03″, its rings 42.00″; Jupiter, 47.88″. The ” symbol means seconds of arc (1/3600th of a degree.) Created using Cartes du Ciel (Sky Charts).
Planets and the Moon at sunset and sunrise of a single night, starting with sunset on the right on September 15, 2021. The night ends on the left with sunrise on the 16th. Click on the image to enlarge. Created using my LookingUp program.
09/14/2021 – Ephemeris – Lack of spacesuits just one of Artemis problems of getting humans back to the Moon
This is Ephemeris for Tuesday, September 14th. Today the Sun will be up for 12 hours and 33 minutes, setting at 7:54, and it will rise tomorrow at 7:21. The Moon, 1 day past first quarter, will set at 12:53 tomorrow morning.
The bright gibbous Moon is a feast for binoculars or a telescope. However, the speed-up plan to land crews on the Moon in 2024 proposed by the former President is not appearing to pan out. One major item is space suits, which must be more rugged and impervious to the Lunar regolith, or soil, whose grains are tiny, angular and sharp, and get into everything, and can destroy spacesuit joints. NASA has been working on them for 14 years, and by itself could cost a billion dollars. They might be ready by 2025. Besides delays to the SLS rocket, the contract with SpaceX to furnish a lunar lander is now tied up in litigation by one of the contract losers. NASA’s trying to land humans on the Moon on one half of one percent of the Federal budget, plus do everything else it does.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT – 4 hr). They may be different for your location.
Addendum
Prototype lunar spacesuit. xEMU stands for Exploration Extravehicular Activity Mobility Unit. Of course. This uncredited image is from slashgear.com in 2019, so I wouldn’t believe the date. These xEMU suits may eventually cost a billion dollars to develop and produce. Click on the image to enlarge it.
09/13/2021 – Ephemeris – The Greeks knew the size and shape of the Earth and estimated the distance to the Moon a long time ago
This is Bob Moler with Ephemeris for Monday, September 13th. Today the Sun will be up for 12 hours and 37 minutes, setting at 7:56, and it will rise tomorrow at 7:20. The Moon, at first quarter today, will set at 11:57 this evening.
The Ancient Greeks used lunar eclipses to determine that Earth is a sphere, and worked on determining the distance to the Moon. From ancient times, the Greeks knew that an eclipse of the Moon was caused by the Earth’s shadow falling on the Moon. Since the Earth’s shadow was always circular, no matter where the Moon was in the sky during an eclipse, the Earth must be a sphere since that’s the only three-dimensional body that always casts a circular shadow. They also used the size of the Earth’s shadow to estimate the distance to the Moon. The lunar distance, on average, is 60.8 times the Earth’s radius away. The first estimates were about one third of that. Hipparchus in the 2nd century BC got much closer. It got even better from there.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT – 4 hr). They may be different for your location.
Addendum
Partial Lunar Eclipse showing circular arc of the Earth’s shadow. Taken 04:15 UT August 17, 1970. Credit: the author.
The size of the Earth was unknown until Eratosthenes did in 240 BC. He came up with the circumference of the Earth to a fairly high degree. The Circumference is equal to the radius of a sphere or circle by 2πr.
