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09/22/2022 – Ephemeris – Autumn will begin this evening

September 22, 2022 Comments off

This is Ephemeris for Thursday, September 22nd. Today the Sun will be up for 12 hours and 10 minutes, setting at 7:40, and it will rise tomorrow at 7:31. The Moon, 3 days before new, will rise at 4:46 tomorrow morning.

The season of fall is about to, ah well, fall upon us and in a few weeks so will the leaves. At 9:04 this evening (1:04 UT tomorrow) the Sun will cross the celestial equator heading south. The celestial equator is an imaginary line in the sky above the earth’s equator. At that point, the Sun will theoretically set at the North Pole and rise at the South Pole. The day is called the autumnal equinox and the daylight hours today is 12 hours and 10 minutes instead of 12 hours exactly. That’s due to our atmosphere and our definition of sunrise and sunset. The reason for the cooler weather now and the cold weather this winter is that the length of daylight is shortening, and the Sun rides lower in the sky, spreading its heat over a larger area, thus diluting its intensity.

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

Sun's path on the equinox for TC-Interlochen

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. Note that the Sun rises due east and sets due west. Created using my LookingUp program.

Sunrise on the autumnal equinox

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.

Autumnal equinox from space

Image from the Earth Polychromatic Imaging Camera (EPIC) on NOAA’s DSCOVR satellite in halo orbit around the Earth-Sun L-1 point 1 million miles sunward from the Earth on the autumnal equinox of 2016. North America is in the upper right of the globe.

Earth's position at the solstices and equinoxes

Earth’s position at the solstices and equinoxes. This is an not to scale oblique look at the Earth’s orbit, which is nearly circular. The Earth is actually farthest from the Sun on July 4th. Click on the image to enlarge. Credit: ESO (European Southern Observatory), which explains the captions in German and English.

09/08/2022 – Ephemeris – We are going to have an early Harvest Moon this year

September 8, 2022 Comments off

This is Bob Moler with Ephemeris for Thursday, September 8th. Today the Sun will be up for 12 hours and 53 minutes, setting at 8:06, and it will rise tomorrow at 7:14. The Moon, 2 days before full, will set at 5:54 tomorrow morning.

We are going to have an early Harvest Moon this year, on the early morning of Saturday the 10th, this Saturday coming up. The Harvest Moon is the closest full moon to the autumnal equinox, which is on the 22nd. The earliest a Harvest Moon can fall is on the 8th of September. The reason that the Harvest Moon is so famous is that at sunset the Moon’s path, in the sky, is shallow to the horizon. So it rises much less than its average 50 minutes later each night. This effectively lengthens the amount of useful twilight, allowing more time to harvest the crops. It compensated for the rapid retreat of the daylight hours this time of year. It’s not so important now, but back before electric lights it definitely was.

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

Harvest Moon Rising ala The Harvest Moon rising as seen in StellariumStellarium

The Harvest Moon rising as seen in Stellarium. The planetarium program Stellarium, which I use a lot, also colors the rising and setting Moon and Sun. It also reproduces the effect of atmospheric refraction, which makes objects close to the horizon look higher than they are. Thus, extended objects close to the horizon appear squashed a bit vertically.

 

The Harvest Moon Effect diagram

The Harvest Moon effect is a phenomenon where the Moon’s nightly advance in rising times become much shorter than the average 50 minutes. This has the effect of extending the bright part of twilight for up to a week near the Harvest Moon. Complicating effects this year are the fact that the Harvest Moon is a supermoon, being a bit brighter than normal, and also moving faster than normal, negating the harvest moon effect somewhat. The Moon’s perigee was on the 7th, so the Moon is slowing down*, which shows in the delay numbers. Also helping to shorten the delay is that the path of the Moon is a bit shallower than the ecliptic. The Moon is south of the ecliptic, heading northward to its ascending node.

The Moon moves fastest in its orbit at perigee, and its slowest at apogee, at its farthest from the Earth.

08/15/2022 – Ephemeris – Saturn moves into the evening sky

August 15, 2022 Comments off

This is Ephemeris for Monday, August 15th. Today the Sun will be up for 14 hours and 3 minutes, setting at 8:48, and it will rise tomorrow at 6:46. The Moon, halfway from full to last quarter, will rise at 10:55 this evening.

Yesterday, Saturn was in opposition to the Sun. I’m not implying an argument here. Opposition is when a planet is opposite the Sun in the sky, so it rises around sunset and sets around sunrise. That means Saturn is the closest it can get to us this year. For the record, that’s 836 million miles (1,345 million kilometers). Saturn will first appear tonight in the southeast when it gets dark enough, say around 9:15 to 9:30 pm. It is in the constellation of Aquarius now. It is moving northeastward in our skies, or it would if it weren’t at opposition, and moving backward or retrograde as the Earth is, in effect, lapping Saturn in our eternal race around the Sun. Saturn’s rings appear to slowly get skinnier as the planet moves to an equinox, where its rings, which orbit its equator, will tilt edgewise to the Sun, and the Earth in 2025.

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

Display of the solar system out to Saturn, with added line showing Saturn’s location opposite the Sun from the Earth. This make the Earth nearly in line from the Sun to Saturn. This is the time that Saturn would be closest to the Earth. Credit: my LookingUp app. I wanted to use NASA’s Eyes, but there were too many interplanetary spacecraft near the Earth. The Earth was crowded out by spacecraft labels.

Saturn's rings change.

How the appearance of the rings change as Saturn orbits the Sun. Credit: NASA Hubble.

08/04/2022 – Ephemeris – The Late Heavy Bombardment

August 4, 2022 Comments off

This is Bob Moler with Ephemeris for Thursday, August 4th. Today the Sun will be up for 14 hours and 32 minutes, setting at 9:04, and it will rise tomorrow at 6:33. The Moon, 1 day before first quarter, will set at 12:19 tomorrow morning.

The face of the Moon is nearly half uncovered from our point of view, with first quarter occurring at 7:06 tomorrow morning. It shows the string of lunar seas, those gray areas on the moon which lead to the terminator, the Moon’s sunrise line. There are even more and larger seas on the east half of the Moon, as we see it. Most were created about 3.9 billion years ago by asteroid strikes. The same thing happened to the Earth, but plate tectonics destroyed the evidence. Not so on the Moon. The result, many planetary scientists think, was the Late Heavy Bombardment, caused by the changing orbits of mainly Saturn, Uranus and Neptune disrupting the smaller asteroids, and sending them careening through the solar system.

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

Tonights Moon with labels

First quarter Moon with prominent seas labeled. Created using Stellarium, GIMP and LibreOffice.

Nice model infographic

Nice model infographic: Evolution of the solar system. Step 6, with the exchange of Neptune’s and Uranus’ orbits, cause the Late Heavy Bombardment. The Nice model isn’t that nice. It’s named for Nice, France, the city where the model was first developed. The original on the web page was smaller. I enlarged it and sharpened it a bit, so it’s more readable. Credit: Nora Eisner.

The above infographic is from the blog post at https://blog.planethunters.org/2019/04/29/formation-of-our-solar-system/ by Nora Eisner.

07/28/2022 – Ephemeris – Why don’t we have solar eclipses every new moon?

July 28, 2022 Comments off

This is Bob Moler with Ephemeris for Thursday, July 28th. Today the Sun will be up for 14 hours and 48 minutes, setting at 9:13, and it will rise tomorrow at 6:25. The Moon is new today, and won’t be visible.

The actual time when the moon is New will be 1:55 pm. The Moon will not eclipse the Sun this time. Why? Because the Moon is nearly 5 degrees, or 10 moon-diameters, north of the Sun. If the Moon orbited the Earth nearly in the same plane that the Earth orbited the Sun, we could have solar eclipses for somewhere on the Earth every new moon. As it is, the Moon orbits the Earth with about a 5-degree tilt to the Earth’s orbit of the Sun. So we get eclipse opportunities of eclipses about one in six new moons for solar eclipse and about the same for full moons and lunar eclipses. Of course, one has to be at the proper location to see them. If the Moon orbited the Earth over the Earth’s equator, like many other moons of other planets, eclipses would be much more rare and only occur around the equinoxes.

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

The Sun and Moon at New Moon 7/28/2022

The Sun and Moon at New Moon at 1:55 pm today, seen as if the Earth had no atmosphere and one could see the Sun and stars at the same time. The orange line is the path of the Sun in the sky, called the ecliptic. The red line is the orbit of the Moon. Created using Stellarium.

Sun and Moon's orbits on the celestial sphere

Earth centered (geocentric) diagram of the heavens called the celestial sphere, showing the apparent orbits of the Sun and Moon. The Moon’s orbit has about a 5-degree tilt (exaggerated here) to the Sun’s apparent orbit, which we call the ecliptic. Solar eclipses occur when the new moon and Sun are near a node. Lunar eclipses occur when the full moon and Sun are near opposite nodes. My diagram.

The orbit of the Moon precesses, so the line of the nodes regresses, that is slowly rotating clockwise, backwards to the motion of the Sun and Moon (and all the rest of the planets), one rotation in 18.6 years.

07/08/2022 – Ephemeris – Polaris the North Star

July 8, 2022 Comments off

This is Ephemeris for Friday, July 8th. Today the Sun will be up for 15 hours and 24 minutes, setting at 9:29, and it will rise tomorrow at 6:06. The Moon, 2 days past first quarter, will set at 2:17 tomorrow morning.

The bright star Polaris is a very important star. It is also known as the North Star and the Pole Star. Its unique position is nearly directly at the zenith at the Earth’s North Pole, making it a very important navigational star. It’s about 40 minutes of arc, or about one and a third Moon diameters away from the extension of the Earth’s axis into the sky. As a rule of thumb, its angular altitude above the northern horizon is approximately one’s latitude, and it stands about at the due north compass point. Polaris is found using the Big Dipper, using the two stars at the front of the dipper bowl to point to it. It’s located at the tip of the handle of the very dim Little Dipper which, this time of year in the evening, appears to be standing on its handle.

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

Polaris finder and location animation. Three frames: visual appearance in the sky, lines of the asterisms of the Big and Little Dippers, addition of the equatorial grid of celestial coordinates analogous to longitude and latitude on the Earth. The right ascension (like longitude) lines converge over the Earth’s North Pole, with Polaris close by. Created using Stellarium and GIMP.

The two stars at the front of the Big Dipper’s bowl, at the bottom of the dipper as it appears now in the evening, point to Polaris near the 11-hour right ascension line. Right ascension, though the same as earthly longitude, is measured in hours, rather than degrees. An hour equals 15 degrees, making 24 hours equal 360 degrees.

06/13/2022 – Ephemeris – What’s a near rectilinear halo orbit?

June 13, 2022 Comments off

Since I wrote and recorded this program on June 5th, the launch date has been postponed from June 13th. No new launch date has, been selected, though the launch window extends through the 22nd for this month’s attempt.

This is Ephemeris for Monday, June 13th. 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 before full, will set at 5:41 tomorrow morning.

Last Friday I talked about the CAPSTONE mission to a near rectilinear halo orbit of the Moon, which, when I recorded this, could launch as soon as today. Anyway, what is a near rectilinear halo orbit? And why is it so special? It is a long looping orbit that comes quite close to the Moon over its North Pole, and very far away over the South Pole of the Moon. Instead of going behind the Moon from the Earth’s perspective, the orbit will always be face-on to the Earth. It will require occasional tweaks to keep it that way, just like the James Webb Space Telescope needs occasional tweaks to keep it in orbit of the Earth-Sun L2 point. That way a lander, or base at the South Pole of the Moon, can keep in communication with the Lunar Gateway for all but a couple of hours each week. Why the South Pole? That’s where the water ice is!

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

NASA CAPSTONE pathfinder for gateway orbit

NASA CAPSTONE* pathfinder for gateway orbit is to be launched this year by Rocket Lab to prove out the near rectilinear orbit for the Lunar Gateway.

The orbit will pass 1,900 miles (3,000 km) above the Moon’s North Pole and out to 43,000 miles (70,000 km) above the South Pole. The period of such an orbit is about 6.5 days. Landing attempts at the South Pole of the Moon will begin as the lander departs the Orion spacecraft or the Lunar Gateway space station and begins its descent over the North Pole.

06/06/2022 – Ephemeris – In June we spend our evenings in the twilight zone

June 6, 2022 Comments off

This is Ephemeris for Monday, June 6th. Today the Sun will be up for 15 hours and 27 minutes, setting at 9:25, and it will rise tomorrow at 5:57. The Moon, 1 day before first quarter, will set at 2:28 tomorrow morning

We spend our evenings after sunset during the months of June and July, pretty much in the twilight zone. Actually, it’s just twilight. There are three twilight periods: Civil, Nautical and Astronomical. In the evening, civil twilight lasts from sunset to when the Sun drops to 6 degrees below the horizon. The scene around is still quite bright, but car headlights still need to be on. Nautical twilight lasts until the Sun is 12 degrees below the horizon. Bright stars and the horizon are visible for sextant use. After that is astronomical twilight until the Sun is 18 degrees below the horizon, when the stars become more and more numerous. After that it’s officially dark, Moon permitting. This time of year we’re lucky to get three and a half hours of darkness.

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

Twilight example for June 6, 2022

Daylight, twilight, and dark example for June 6, 2022. This graph is centered on midnight. Light blue is daylight, while the three darker shades of blue denote the three twilight periods of civil, nautical and astronomical. The yellow lines show when the Sun, Moon and planets are visible. Dark of night is relegated only to the morning hours, thanks to the season, daylight saving time and our location 43 minutes west of our standard time meridian. The chart is produced by the app Cartes du Ciel (Sky Charts).

Categories: Concepts, Observing, Twilight

04/14/2022 – Ephemeris – The Moon appears to wobble

April 14, 2022 Comments off

This is Bob Moler with Ephemeris for Thursday, April 14th. Today the Sun will be up for 13 hours and 26 minutes, setting at 8:26, and it will rise tomorrow at 6:58. The Moon, 2 days before full, will set at 6:43 tomorrow morning.

Tonight’s Moon is nearly full, only two days to go. In the evening, the little dark sea on the Moon called Mare Crisium or Sea of Crises is near the top of the Moon. Now it’s real close to the top edge of the Moon, what astronomers call the limb. Ten days ago, when the Moon was a crescent, Mare Crisium was farther from the limb. This wobbling motion over the lunar month is called libration. This occurs because the Moon has an elliptical orbit of the Earth and speeds up when it’s closer and slows when it’s farther away. However, the Moon’s rotation is constant due to its angular momentum. The period of the revolution of the Moon around the Earth exactly matches its rotation. When the Moon is farthest, its rotation gets a bit ahead of its revolution around the Earth. When closest, the rotation lags a bit. So it appears to wobble over the lunar month.

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

The Moon as it should appear at 10 pm tonight

The Moon as it should appear at 10 pm tonight, April 14, 2022. Note the position, relative to the Moon’s limb, of the small lunar sea called Mare Crisium near the top of the Moon. Mare Crisium means Sea of Crises. It is quite close to the limb tonight. In the animation below, shown without the phase, the orientation of the Moon is not changed by the position in the sky or the latitude of the observer. Created using Stellarium.

Demonstration of libration via animation

A demonstration of libration by viewing the position of Mare Crisium. Also note that there is also a libration in the Moon’s latitude, causing a north-south nodding. Another good indicator of libration is the dark floored crater called Grimaldi on the left side of the Moon, which is not visible tonight in the image at the top. But it will appear in sunlight tomorrow night. Created using Virtual Moon Atlas.

03/17/2022 – Ephemeris – We have 12 hours of daylight and night today, three days early

March 17, 2022 Comments off

This is Ephemeris for St. Patrick’s Day, Thursday, March 17th. Today the Sun will be up for 12 hours even, setting at 7:51, and it will rise tomorrow at 7:49. The Moon, 1 day before full, will set at 8:20 tomorrow morning.

Why is this the day of equal day and night, when the vernal equinox, which means “equal night” is still three days away? The difference is due to our atmosphere and our definition of sunrise and sunset. Our atmosphere makes objects near the horizon appear higher than they actually are, which hastens sunrise and retards sunset. Also, the instant of sunrise and sunset is when the top of the sun appears to touch the horizon, rather than when the Sun bisects the horizon. Plus, it’s moving about a degree a day (twice its diameter) against the stars. So by the time of the equinox, on Sunday the 20th, the time between sunrise and sunset will have progressed to 12 hours and 9 minutes. But it was close enough for the ancients who coined the term.

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

This works for the Interlochen area. It may be a different by a day for other locations, but for the northern latitudes, it will be before the true equinox day, March 20th here.

Atmospheric Refraction

How the atmosphere bends the light of the Sun or Moon rising or setting to appear higher than it actually is. S is the actual position of the Sun, S’ is the apparent position of the Sun. The blue line is the observer O’s horizon. The gray line is the actual, though much exaggerated, light path bent or refracted by the Earth’s atmosphere. The black line is the apparent sight line to the Sun. Credit Francisco Javier Blanco González, 2017