Archive
Ephemeris: 09/29/2025 – First attempts to measure the distance to the Sun
This is Ephemeris for Monday, September 29th. Today the Sun will be up for 11 hours and 47 minutes, setting at 7:26, and it will rise tomorrow at 7:39. The Moon, at first quarter today, will set at 11:38 this evening.
The first quarter Moon tonight reminds me of how the Greeks used the quarter Moon to attempt to determine the distance to the Sun. The idea was to determine when the Moon was exactly at first or last quarter, so the angle of the Sun-Moon-Earth was exactly 90°. The next thing to do was to measure the actual angle between the Sun and the Moon at that instant. It’s a difficult observation. Aristarchus tried and got a result that the Sun was about 19 times the distance of the Moon. The Sun-Earth-Moon angle he got was 87°. Hipparchus measured the Moon to be 60 earth radii away which is near the Moon’s actual distance from the Earth. In actuality the Sun is about 400 times the distance to the Moon.
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: 09/26/2025 – Pegasus rising
This is Ephemeris for Friday, September 26th. Today the Sun will be up for 11 hours and 56 minutes, setting at 7:32, and it will rise tomorrow at 7:36. The Moon, 3 days before first quarter, will set at 9:19 this evening.
Saturn is well up in the east southeastern sky at 10 PM. This is the brightest “star” in that direction. Below it, at about the 4 o’clock position from it is Fomalhaut, which I sometimes call the loneliest star in the sky, because without Saturn or another planet in that direction it seems pretty much alone low in the south. Above and a bit to the left of Saturn is the Great Square of Pegasus A four star group standing on one corner which is the body of Pegasus the flying horse. Between Saturn and the Great Square is a faint and small circle of five or six stars called the Circlet, an asterism, which is a loop around one of the fish of Pisces the fish. Pisces is two fish held together by a long rope.
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: 09/25/2025 – Today is Equilux day
This is Ephemeris for Thursday, September 25th. Today the Sun will be up for 12 hours to the minute, setting at 7:33, and it will rise tomorrow at 7:35. The Moon, halfway from new to first quarter, will set at 8:50 this evening.
Today is Equilux day it’s the day we have exactly 12 hours of sunshine, and 12 hours of night. But isn’t that what we had three days ago on the equinox? That’s what equinox means. But, it only works on the equinox if the Earth didn’t have an atmosphere, and we decided that sunrise and sunset was when the middle of the sun crosses the horizon. The actual definition of sunrise and sunset is when the top of the Sun hits the horizon. So it rises a little earlier and so it’s a little bit later than it does geometrically. Also, the Earth has an atmosphere which causes the Sun to be higher in the sky when it’s closer to the horizon, which prolongs daylight. Check the weather app on your smartphone for your own personal sunrise and sunset times.
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: 09/24/2025 – Taking our weekly look at the whereabouts of the naked-eye planets
This is Ephemeris for Wednesday, 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, 3 days past new, will set at 8:27 this evening.
Let’s take our weekly look at the whereabouts of the naked-eye planets. Saturn now rises about sunset in the east. It was in opposition from the Sun last Sunday. In a telescope Saturn sports a very thin ring, 1.7° from being edge on. Saturn has an axial tilt of about 27°, so it has seasons like the Earth and since its rings are over its equator, when they go edge on to the Sun, it is an equinox for it, which happened this past May. Earth, being close to the Sun, see nearly the same thing. Now the ring angle for us will decrease to about a third of a degree by November 23rd before increasing. By 6:30 AM Jupiter will appear high in the east-southeast, under the stars of Gemini. With brighter Venus low in the east below the star Regulus in Leo.
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: 09/23/2025 – Neptune is at opposition today
This is Ephemeris for Tuesday, 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, 2 days past new, will set at 8:07 this evening.
I normally don’t talk about observing things in the sky unless it can be seen with the naked eye, or can be seen in binoculars which are near stars that are visible to the naked eye. So I rarely talk about the planet Neptune. The last time was eight years ago and for the same reason as today. Neptune is in opposition with the Sun. That is, it is exactly opposite to the Sun in the sky, rising at sunset and setting at sunrise. Not only that, today is the 179th anniversary of the date it was discovered in 1846. Interestingly, it was first seen by Galileo. In one of his drawings of Jupiter and its moons that he made with his small telescope, there happened to be a background star that he recorded. That star turned out to be Neptune.
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: 09/22/2025 – Autumn befalls us today
This is Bob Moler with Ephemeris for Monday, 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, 1 day past new, will set at 7:50 this evening.
This is it! Summer will end at 2:20 this afternoon, 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 night 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 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: 09/19/25 – A different take on Cygnus the swan
This is Ephemeris for Friday, September 19th. Today the Sun will be up for 12 hours and 18 minutes, setting at 7:45, and it will rise tomorrow at 7:27. The Moon, 2 days before new, will rise at 6:06 tomorrow morning.
Yesterday I looked the star in the beak of the constellation of Cygnus the swan and the informal constellation or asterism made from most of its stars the Northern Cross. Cygnus is the official International Astronomical Union constellation name. However, the indigenous Anishinaabe people of our area had another bird in mind when seeing these stars, which are now fairly high in the east in the evening: Ajijaak, (pronounced a-ji-jock) a Sand Hill crane. While the swan is flying, neck outstretched to the south through the Milky Way, the crane is flying northward with its long legs trailing behind. The bright star Deneb is at his head. Where I live I see more cranes than swans these days and hear their creaking-door-like calls, and see a pair from time to time in a field south of where I live.
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: 09/18/2025 – A fine example of different star colors
This is Bob Moler with Ephemeris for Thursday, September 18th. Today the Sun will be up for 12 hours and 21 minutes, setting at 7:47, and it will rise tomorrow at 7:26. The Moon, 3 days before new, will rise at 4:56 tomorrow morning.
Alberio is the name of the star that is in the head of the constellation of Cygnus the swan, which is almost overhead these evenings. It is also at the foot of the asterism or informal constellation of the Northern Cross. To the naked eye Alberio looks like a single star, however even in small telescopes its true nature is revealed. It’s a double star whose individual star colors are strikingly different Its brightest star is yellow, and the dimmer star is blue. While star colors are subtle, these two, due to their apparent closeness, make an obvious color contrast. Unlike what your interior decorator says: In stars, blue is hot, yellow, orange and red are cool. The two stars are too far apart to be considered a binary star system, but appear to move together in space. It is what is called an optical double, though they’re both around 430 light years away.
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: 09/17/2025 – Taking a look at the whereabouts of the naked-eye planets
This is Ephemeris for Wednesday, 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 past last quarter, will rise at 3:44 tomorrow morning.
Let’s take our weekly look at the whereabouts of the naked-eye planets. Saturn now rises at 8:01 PM in the east. In a telescope Saturn sports a very thin ring, 2° from being edge on. Saturn has an axial tilt of about 27°, so it has seasons like the Earth and since its rings are over its equator, when they go edge on to the Sun, it is an equinox for it, which happened in May. Earth being close to the Sun and moving, see nearly the same thing. Now the ring angle for us will decrease to about a third of a degree by November 23rd before increasing. By 6 AM Jupiter will appear above the brighter Venus in the Eastern sky and among the stars of Gemini. Venus and Jupiter will have the waning crescent Moon between them tomorrow morning.
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: 09/16/2025 – NASA’s announcement of discovery that might be evidence for past life on Mars
This is Ephemeris for Tuesday, September 16th. Today the Sun will be up for 12 hours and 27 minutes, setting at 7:50, and it will rise tomorrow at 7:24. The Moon, 2 days past last quarter, will rise at 2:29 tomorrow morning.
Turning from the current crisis last week, NASA last Thursday, announced the detection of Martian elements and minerals that could have been associated with life, when Mars was more hospitable for life billions of years ago. This was discovered by the Perseverance Rover in a dry (of course) river valley in the delta, where the river once flowed into Jezero crater. Further chemical analysis will have to wait until samples of the rock are returned to the earth. Perseverance has made core samples of the rock in question, but the problem is the mission to retrieve the samples has yet to be fully defined and may be canceled. This is due to a change in strategy for NASA for the de-emphasis of science by the current US administration.
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
Falls workspace on sol 1217. The light-toned layered block contains the Cheyava
Falls natural surface target, the Apollo Temple abrasion and the Sapphire
Canyon core sample location. The Sapphire Canyon sample was collected
from approximately the same location as Cheyava Falls after analysis of the
target was completed. The darker-toned granular block contains
the Steamboat Mountain abrasion. Downhill is to the left on this image.
Mastcam-Z enhanced color RGB vertical projection mosaic from sol 1217,
sequence zcam09264, acquired at 110-mm focal length. Scale bar, 10 cm.
Credit: NASA/JPL-Caltech/ASU/MSSS. Caption edited from the September 11, 2025 Nature Article Redox-driven mineral and organic associations in Jezero Crater, Mars.
Link to the article: Redox-driven mineral and organic associations in Jezero Crater, Mars https://www.nature.com/articles/s41586-025-09413-0
Link to the paper: Hurowitz, J.A., Tice, M.M., Allwood, A.C. et al. Redox-driven mineral and organic associations in Jezero Crater, Mars. Nature 645, 332–340 (2025). https://doi.org/10.1038/s41586-025-09413-0
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