Archive
Ephemeris: 07/29/2025 – Altair, the nearest Summer Triangle star
This is Ephemeris for Tuesday, July 29th. Today the Sun will be up for 14 hours and 45 minutes, setting at 9:11, and it will rise tomorrow at 6:27. The Moon, 3 days before first quarter, will set at 11:23 this evening.
The southernmost star of the Summer Triangle is Altair, high in the east-southeast. The other two stars of the triangle are Vega nearly overhead in the east, and Deneb high in the east-northeast. Altair is the nearest of the three at a distance of 16.7 light years away. One light year is nearly 6 trillion miles. Altair is 10 times the brightness of the Sun. If seen at Altair’s distance, the Sun would only be as bright as one of the two stars that flank it. What is rather different about Altair is its rapid rotation. While it’s almost twice the sun’s diameter, it rotates once in about 9 hours, The CHARA Interferometer at Mt. Wilson has actually imaged its squashed disk in the infrared. Our Sun’s a slow poke, taking nearly a month to rotate once.
The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EDT, UT – 4 hours) unless stated otherwise. Times will be different for other locations.
Addendum
Ephemeris: 07/28/2025 – Deneb, a truly brilliant star
This is Ephemeris for Monday, July 28th. Today the Sun will be up for 14 hours and 48 minutes, setting at 9:12, and it will rise tomorrow at 6:26. The Moon, halfway from new to first quarter, will set at 11:06 this evening.
This evening when it gets dark enough the bright star Deneb in Cygnus the swan will be high in the east-northeast. Deneb is the dimmest star of the summer triangle. Of the other stars of the triangle, Vega is higher in the east, while Altair is lower in the southeast. Deneb’s apparent magnitude, or brightness as seen from Earth, is deceptive. Its vast distance of possibly 2,600 light years is over 100 times the distance of Vega. If brought as close as Vega, Deneb would be almost as bright as the quarter moon. It is possibly as bright as 200 thousand Suns; and a huge star, possibly as large as half the diameter of Earth’s orbit. For all this, it is only 19 to 25 times the mass of the Sun.
The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EDT, UT – 4 hours) unless stated otherwise. Times will be different for other locations.
Addendum
Ephemeris: 07/24/2025 – Vega, nearing the zenith in the evening now
This is Ephemeris for Thursday, July 24th. Today the Sun will be up for 14 hours and 56 minutes, setting at 9:17, and it will rise tomorrow at 6:21. The Moon is new today, and won’t be visible.
Vega, in the constellation Lyra the harp, is the highest bright star In the east and brightest star of the Summer Triangle also rising in that direction. It is an important and much studied star, first as a standard for brightness for the magnitude scale at almost exactly zero. In 1983 the Infrared Astronomy Satellite, IRAS, discovered an excess of infrared radiation coming from the star. It seems now that there are two orbiting rings around the star, an inner warm ring and an outer cold ring. This is somewhat like the two disks of material the Sun has: The asteroid belt between Mars and Jupiter and the Kuiper belt, beyond Neptune. No planets have yet been discovered around Vega, but I wouldn’t bet against it.
The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EDT, UT – 4 hours) unless stated otherwise. Times will be different for other locations.
Addendum
Ephemeris: 06/23/2025 – The Big Dipper is tearing itself apart
This is Ephemeris for Monday, June 23rd. Today the Sun will be up for 15 hours and 34 minutes, setting at 9:32, and it will rise tomorrow at 5:58. The Moon, 2 days before new, will rise at 4:33 tomorrow morning.
The usual impression is to think that the stars of a constellation are actually located close together. This is usually not true. The stars of a constellation can be at vastly different distances. The Big Dipper is different. The five stars, excepting the two end stars of the dipper and 12 other dimmer stars in the general area are of similar distance and have the same motion through space. The group is called the Ursa Major Moving Cluster or Ursa Major Association, and is moving about 9 miles per second relative to the solar system to the east. An association is a rather loose, sparse star cluster. This association lies about 80 light years away. If it were five and a half times farther away, it would be the same distance as the Pleiades.
The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EDT, UT – 4 hours) unless stated otherwise. Times will be different for other locations.
Addendum
Ephemeris: 06/12/2025 – Another look at the star Spica
This is Ephemeris for Thursday, June 12th. Today the Sun will be up for 15 hours and 32 minutes, setting at 9:29, and it will rise tomorrow at 5:56. The Moon, 1 day past full, will rise at 11:20 this evening.
In the south-southwest at 11 PM is the bright star Spica in Virgo the virgin. Arcturus, high in the south, is much brighter than Spica and has an orange tint to Spica’s bluish hue. In fact, Spica is the 15th brightest and the bluest of the 21 first magnitude stars. That means that it has a really hot surface temperature. Spica is actually two stars in a tight 4-day orbit of each other. They are both reasonably matched in mass and brightness. I found that out once photographing a lunar eclipse near Spica, The star came out very blue. The twin stars of Spica are 250 light years away. I’m glad the stars are young now. They will have a very interesting future as they age and interact in the next few million years.
The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EDT, UT – 4 hours) unless stated otherwise. Times will be different for other locations.
Addendum
Ephemeris: 05/16/2025 – Two bright stars, alike but different
This is Bob Moler with Ephemeris for Friday, May 16th. Today the Sun will be up for 14 hours and 52 minutes, setting at 9:05, and it will rise tomorrow at 6:12. The Moon, halfway from full to last quarter, will rise at 1:21 tomorrow morning.
Two stars which I’ve been known to confuse in the twilit sky in late spring and late summer are Arcturus and Vega. These stars have about the same brightness and in some lists of stars swap places between the 4th and 5th brightest stars in the sky. Arcturus is an orangish star, now high in the southern sky in the evening, while Vega is a pure white star which to my eyes tends towards a little tinge of blue. Vega is high in the south, actually almost overhead, in the late summer. Now, in mid to late Spring Vega is rather low in the northeastern sky. Arcturus is a cool star on the outside which has depleted the hydrogen in its core and is a red giant star, while the younger and more massive Vega, still turning hydrogen into helium.
The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EDT, UT – 4 hours) unless stated otherwise. Times will be different for other locations.
Addendum
Ephemeris: 05/13/2025 – Arcturus, extragalactic visitor?
This is Ephemeris for Tuesday, May 13th. Today the Sun will be up for 14 hours and 45 minutes, setting at 9:02, and it will rise tomorrow at 6:15. The Moon, 1 day past full, will rise at 10:39 this evening.
The bright orange star high in the southeast at 10 PM is Arcturus. Remember: Follow the arc of the handle of the Big Dipper to find Arcturus. It is an interesting star in many respects. Arcturus is somewhat more massive than the Sun and a bit older. It is starting its red giant phase having run out of hydrogen in its core and starting to use helium as its heat source, transmuting it into carbon and other elements. It has a very high velocity with respect to the Sun of about 100 kilometers per second. Arcturus is thought to be, by some astronomers, part of the remnants of a dwarf galaxy that collided with the Milky Way, and has now been assimilated. So Arcturus isn’t from around here.
-The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EDT, UT – 4 hours) unless stated otherwise. Times will be different for other locations.
Addendum
Ephemeris: 04/25/2025 – The star that opened a World’s Fair
This is Ephemeris for Arbor Day, Friday, April 25th. Today the Sun will be up for 13 hours and 59 minutes, setting at 8:40, and it will rise tomorrow at 6:39. The Moon, 2 days before new, will rise at 5:50 tomorrow morning.
High in the eastern sky at 10 tonight can be found the kite shaped constellation of Boötes the herdsman, chasing or herding the Great Bear Ursa Major of which the Big Dipper is the hind end, across the sky. The bright star at the base of the kite is the 4th brightest nighttime star, Arcturus. It can be found and name remembered by first locating the Big Dipper and by following the arc or curve of the handle to Arcturus. This star is an orange-colored giant star, 37 light years away. Its light was used to open the 1933 Chicago Worlds Fair believing its light left the star in 1893 the year of the previous Chicago Worlds Fair. It turns out that Arcturus is 3 light years closer than what they thought.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT – 4 hours). Times will be different for other locations.
Addendum
For more on how they did it go to: https://chicagology.com/centuryprogress/1933fair54/
Ephemeris: 04/21/2025 – Follow the spike to Spica
This is Ephemeris for Monday, April 21st. Today the Sun will be up for 13 hours and 47 minutes, setting at 8:35, and it will rise tomorrow at 6:46. The Moon, 1 day past last quarter, will rise at 4:25 tomorrow morning.
In the south-southeast at 10:30 p.m. is the bright star Spica, which can be found from all the way back overhead to the Big Dipper. Follow the arc of the handle of the Big Dipper to the bright star Arcturus below it in the east-southeast. Then straighten the curve of the arc to a straight spike which points to Spica, the brightest star in Virgo the virgin. Arcturus is much brighter than Spica and has an orange tint to Spica’s bluish hue. In fact, Spica is the bluest of the 21 first magnitude stars. That means that it has a really hot surface temperature. Actually, Spica is really two blue-white stars orbiting each other in 4 days. Spica is 250 light years away. It also was an important star to the ancient Greeks. One temple was built and aligned to its setting point. The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT – 4 hours). Times will be different for other locations.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT – 4 hours). Times will be different for other locations.
Addendum
Ephemeris: 04/14/2025 – Follow the arc to Arcturus
This is Ephemeris for Monday, April 14th. Today the Sun will be up for 13 hours and 27 minutes, setting at 8:27, and it will rise tomorrow at 6:57. The Moon, 2 days past full, will rise at 10:36 this evening.
The fourth or fifth-brightest nighttime star, depending on whose list you see, is now up in the east in the evening. It is Arcturus, a bright star with an orange hue. It can be found otherwise by finding the Big Dipper and tracing out and extending the curve of the handle and “Follow the arc of the handle to Arcturus”, to remember the name of the star and how to find it. Arcturus is about 37 light years from us and is moving quite rapidly across the sky, compared to most stars, though one would not notice it to the naked eye in one’s lifetime. Arcturus is slightly more massive than our Sun, and about 7 billion years old, and is entering its red giant stage of life after using all the hydrogen fuel in its core. Our Sun, being slightly less massive will survive on hydrogen a bit longer.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT – 4 hours). Times will be different for other locations.
Addendum
Bob Moler's Ephemeris Blog 