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04/11/2019 – Ephemeris – How far away is the Moon?
Ephemeris for Thursday, April 11th. Today the Sun will be up for 13 hours and 16 minutes, setting at 8:22, and it will rise tomorrow at 7:04. The Moon, 1 day before first quarter, will set at 3:02 tomorrow morning.
This year, the 50th anniversary of the Apollo 11 first human landing on the Moon, I’ll be talking about some basic facts about the Moon, the Apollo program. The first thing is to realize just how far the Moon is from the Earth. Most diagrams of the Earth and Moon cheat and make them closer than they are. The Greek astronomer Hipparchus in the second century BC got pretty close. The Moon is about 30 times the Earth’s diameter away. If the Earth were represented by a basketball and the Moon by a tennis ball to get their proportional distance correct they would have to be 23 and a half feet (7.16 meters) away from each other, give or take. On average 238,000 miles (383,000 km). It took the Apollo astronauts 3 days cover that distance to get to the Moon.
The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
The Earth and Moon to scale. Click on the image to enlarge. Source Wikipedia.
04/08/2019 – Ephemeris – How to find Polaris, the North Star
Ephemeris for Monday, April 8th. Today the Sun will be up for 13 hours and 7 minutes, setting at 8:18, and it will rise tomorrow at 7:09. The Moon, 3 days past new, will set at 11:54 this evening.
The most useful of the navigation stars for the average person is Polaris, the North Star or Pole Star. It is very close to the point in the sky that the Earth’s axis points to in the north. Currently it is about three-quarters of a degree from the pole, about one and a half moon diameters. In 2110 or thereabouts it will approach to slightly less than a moon diameter from the pole before slowly heading away. Polaris is always closer to true north than a magnetic compass in Michigan. To find it use the two stars in front of the Big Dipper’s bowl to point to it. This time of year the Big Dipper is above Polaris, so the pointer stars, that’s what they are called, point down to it. Polaris is at the end of the handle of the faint Little Dipper. The reason for Polaris’ motion is the slow 26,000 year wobbling of the Earth’s axis, called precession.
The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
Ursa Major and Minor, the Big and Little Dippers. See how the two stars at the front of the bowl point to Polaris. It happens that the pointer stars are close to the 11th hour of right ascension (longitude in the sky). The right ascension lines converge at the north celestial pole, just as the longitude lines converge at the Earth’s north pole. Created using Stellarium.
The year I was born, 1941, Polaris was a whole degree from the celestial north pole.
If you’ve ever wondered why right ascension is in hours instead of degrees it’s because the Earth rotates within the celestial sphere, so it’s easier to keep track of the east-west position in the sky by using a clock that set to gain 3 minutes and 56 seconds a day. Such a clock keeps sidereal (star) time rather than solar (sun) time. One hour equals 15 angular degrees or 4 minutes a degree.
03/21/2019 – Ephemeris – Are day and night really equal at the equinoxes?
Ephemeris for Thursday, March 21st. Today the Sun will be up for 12 hours and 11 minutes, setting at 7:56, and it will rise tomorrow at 7:42. The Moon, 1 day past full, will rise at 8:52 this evening.
What we had yesterday was the vernal equinox, the start of spring. The word equinox means “equal night”. Yesterday’s daylight hours were 12 hours and 8 minutes. What’s with the 8 minutes? The rising or setting Sun is a mirage. The Earth’s atmosphere acts like a lens and makes the Sun appear higher in the sky than when it is when near the horizon. When the bottom edge of the Sun touches the horizon the Sun is actually still completely below the horizon geometrically. If the Earth had no atmosphere sunrises would occur 4 minutes later, and sunsets would occur 4 minutes earlier around here. That would completely correct the 12 hour 8 minutes daylight time of yesterday to 12 hours even.
The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
How the atmosphere bends the light of the Sun or Moon rising or setting to appear higher than it actually is. Credit Francisco Javier Blanco González, 2017.
I took a look at the related atmospheric refraction effect last month: https://bobmoler.wordpress.com/2019/02/19/02-19-2019-ephemeris-the-moon-aint-just-super-near-the-horizon/.
03/18/2019 – Ephemeris – Spring, the full moon and Easter
Ephemeris for Monday, March 18th. Today the Sun will be up for 12 hours and 2 minutes, setting at 7:52, and it will rise tomorrow at 7:48. The Moon, 2 days before full, will set at 7:21 tomorrow morning.
Spring is two days away. In checking my astronomical calendars I noticed an odd thing related to the date of Easter for western churches. If I said that the date of Easter was the first Sunday after the first full Moon after the vernal equinox. I’d be wrong. Even if I replaced vernal equinox with first day of spring, I would still be wrong by ecclesiastical standards. The ecclesiastical vernal equinox is March 21st, no matter what. Plus the full moon date is a tabulated value and not necessarily the astronomical full moon date. This year the astronomical first full moon of spring falls less than 4 hours after the astronomical vernal equinox on March 20th. Therefore Easter will be late this year on April 21st, 4 days earlier than its latest possible date.
The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
03/14/2019 – Ephemeris – Happy Pi Day
Ephemeris for Pi Day, Thursday, March 14th. Today the Sun will be up for 11 hours and 49 minutes, setting at 7:47, and it will rise tomorrow at 7:55. The Moon, at first quarter today, will set at 4:09 tomorrow morning.
Pi day: the first three digits of the mathematical constant π is 3.14 the ratio of a circle’s circumference to its diameter, so March 14th is Pi day. Why do we use the Greek letter pi? Pi is the first letter of the Greek word perimetros which means circumference. The first Pi Day was celebrated in 1988, organized by Larry Shaw at San Francisco’s Exploratorium. There are other lass known pi related days on the calendar. The fraction used in pi approximations is 22/7ths. So July 22nd is Pi Approximation Day. In many formulae π is multiplied by two, or 2π. 2π to two decimal places is 6.28, so it’s Two Pi Day or Tau (τ) Day, June 28th. So whether you like apple, cherry, or pumpkin, have a happy Pi day. https://www.piday.org/pi-facts/
The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
Credit http://www.piday.org.
Check out https://www.piday.org/pi-facts/ for more information on pi.
03/12/2019 – Ephemeris – Does the Moon rotate?
Ephemeris for Tuesday, March 12th. Today the Sun will be up for 11 hours and 43 minutes, setting at 7:44, and it will rise tomorrow at 7:59. The Moon, 2 days before first quarter, will set at 2:00 tomorrow morning.
Since we see only one face of the Moon throughout the month does that mean that the Moon doesn’t rotate? It doesn’t appear to rotate with respect to the Earth, but the Moon rotates through all the stars and constellations of the zodiac in a 27.32 days. That’s called a sidereal month. The lunar month we’re more familiar with is the synodic month, or lunation, that lasts 29.53 days which is the interval between new moons. It is the basis of the Jewish and Islamic calendars. The reason for the locking of the Moon’s rotation and revolution of the Earth, also called synchronous rotation, is the tidal pull of each on the other. The Earth has managed to slow the Moon’s rotation to match its revolution. The Moon is trying to do the same to the Earth with much less success, though every year or two a second is added to the time stream, UTC1 or Greenwich Mean Time, due to the slowing of the Earth’s rotation.
The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
Animation of the Moon orbiting the Earth showing the Moon’s rotation. Credit: NASA’s Scientific Visualization Studio/Ernie Wright, modified and converted to GIF by Bob Moler.
NASA’s Scientific Visualization Studio web page where I got the original for the above video.
03/11/2019 – Ephemeris – The Moon: Dark side, far side, which is it?
Ephemeris for Monday, March 11th. Today the Sun will be up for 11 hours and 40 minutes, setting at 7:43, and it will rise tomorrow at 8:01. The Moon, 3 days before first quarter, will set at 12:54 tomorrow morning.
Tonight the Moon is at its crescent phase, meaning it is slightly closer to the Sun than the Earth is. Most of the Moon we see is in night. Some earth shine may be seen on its night side due to the big nearly full Earth shining on it. I get ticked sometimes when someone who knows better, especially in the media, mentions the dark side of the Moon when they should use the term far side, the part of the Moon that permanently faces away from the Earth. When the Chinese Chang’e 4 spacecraft landed on the far side of the Moon recently many headlines proclaimed that it landed on the dark side of the Moon. The Moon has a night side, as does the Earth, but that changes as the Moon rotates in the sunlight. And the Moon does rotate. It happens to be in sync with its revolution about the Earth.
The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
The crescent Moon tonight at 9 p.m. EDT, March 11, 2019. Created using Stellarium.
Demonstration of the Moon’s crescent phase with the Styrofoam moon ball we use for Project Astro held up to a light off frame to the right. The night side of the ball is illuminated a bit by the translucency of the ball, and the reflection off my hand. Note the roughness of the ball is visible only at the terminator.
The Earth and Moon if seen as a crescent, near side, far side, sunlit side and earth shine. Credit: me.
02/26/2019 – Ephemeris – Mercury at greatest eastern elongation from the Sun tonight
Ephemeris for Tuesday, February 26th. Today the Sun will be up for 11 hours even, setting at 6:26, and it will rise tomorrow at 7:24. The Moon, at last quarter today, will rise at 2:49 tomorrow morning.
This evening the planet Mercury is at its greatest distance from the Sun to the east by an angle of 18 degrees. It’s called greatest eastern elongation from the Sun. Mercury has a very elliptical orbit of the Sun, and right now it’s near its closest to the Sun called perihelion, of 28.6 million miles (46.1 million km). In late winter and spring the ecliptic, the path in the sky that the planets appear near, meets the horizon at a steep angle near sunset, which allows us to see planets near and east of the Sun more easily. The same is true for the planets west of the Sun in the morning in the fall. Southern hemisphere observers see Mercury best when its is at aphelion, farthest from the Sun, 66 percent farther away.
The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
Looking at Mercury at greatest eastern elongation tonight February 26, 2019 from Northern Michigan displaying its orbit with a transparent horizon at sunset. Notice how lopsided the orbit appears, extending farther below the horizon (green line) than above. The yellow line is the ecliptic. Created using Stellarium and GIMP.
The next greatest elongation of Mercury is the western elongation on April 11, 2019. Here we are looking at it from the southern hemisphere, where it’s autumn displaying its orbit with a transparent horizon at sunrise. Notice how lopsided the orbit appears, extending farther above the horizon (green line) than below. The yellow line is the ecliptic. Created using Stellarium and GIMP.
Note the same is true for both northern and southern hemispheres:
Late winter and spring – planets near and east of the Sun are seen more easily after sunset.
Late summer and autumn – planets near and west of the Sun are seen more easily before sunrise.
02/19/2019 – Ephemeris – The Moon ain’t just super near the horizon
Ephemeris for Tuesday, February 19th. Today the Sun will be up for 10 hours and 39 minutes, setting at 6:16, and it will rise tomorrow at 7:35. The Moon, at full today, will rise at 6:28 this evening.
Tonight we will have a super full Moon. It will be the closest full moon of the year, not that you could really tell. The full moon, or waning moon looks large near the horizon when it’s rising. The same is true of waxing moons setting. The same is true of sunrises and sunsets. However if you look closely at the rising or setting Moon or Sun right at the horizon you will notice that it appears a bit squashed. This is due to atmospheric refraction or bending of the light which makes the them appear higher in the sky than they actually are. It is most pronounced near the horizon. In fact by the time the Sun appears to touch the horizon, it is already completely below the horizon, and would appear be if we didn’t have an atmosphere.
The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
The Moon just after moonrise 1 day past full. The Moon appears squashed vertically by about 10% compared to circumscribed circle. Credit Eileen Carlisle.
How the atmosphere bends the light of the Sun or Moon rising or setting to appear higher than it actually is. Credit Francisco Javier Blanco González, 2017
02/18/2019 – Ephemeris – Super Moon, super math
Ephemeris for President’s Day, Monday, February 18th. Today the Sun will be up for 10 hours and 36 minutes, setting at 6:15, and it will rise tomorrow at 7:37. The Moon, 1 day before full, will set at 7:52 tomorrow morning.
Ready for some math this morning? This will be easy, because you will only have to think about it. Let’s say the Earth’s radius is 4,000 miles. That is pretty close to its actual value. OK, it is 3961 miles (6,378 km) at the equator. The Earth is nearly spherical. At 4 this morning the Moon passed perigee, its closest point to the Earth, making tomorrow’s full moon a super-moon. Let’s say it will be at today’s distance of 221,600 miles (356,800 km). That’s center to center. At moon rise or moon set the Moon is near that center to center distance, but if it moves overhead it’s 4,000 miles closer because we are on the Earth facing the Moon. Even though the Moon looks smaller than when it appears on the horizon. It’s an optical illusion that the Moon appears larger when it is rising. Super moon or not.
The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
This illustrates the difference in the Moon’s distance at rising (or setting) versus when it is overhead. BTW today’s perigee puts the Moon at only 56 Earth radii from the Earth. The illustration is mine.
Bob Moler's Ephemeris Blog 