Archive

Archive for the ‘Concepts’ Category

04/27/2021 – Ephemeris – What is the opposite of the Harvest Moon effect?

April 27, 2021 Comments off

This is Bob Moler with Ephemeris for Tuesday, April 27th. Today the Sun will be up for 14 hours and 4 minutes, setting at 8:43, and it will rise tomorrow at 6:36. The Moon, 1 day past full, will rise at 9:43 this evening.

Let’s think back to last fall and the Harvest Moon. The big deal about the Harvest Moon is the Moon lingers, rising or being bright in twilight to help illuminate the harvesters of old by effective lengthening the effects of daylight. The spring bright Moon after full moon rises much later night to night. Six months ago the difference in the rise times of the Moon between the full moon and the next day was 20 minutes. Today the Moon will rise 96 minutes later than it did yesterday. The reason is the beside moving eastward, it is also moving southward to where the Sun was in late fall. So it rises much later each night than it did after full moon last fall. As it is the dark skies are moving to later and later in the evening due to the spring season and daylight time.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

The motion of the moon
The motion of the moon near the Harvest Moon as it rises from night to night. This is looking east at where the Moon will rise, and we’re able to see below the horizon. The celestial equator, a projection of the Earth’s equator on the sky cross the horizon at an angle equal to 90 minus one’s latitude. Around my location that’s 45.5 degrees. The Moon will rise parallel to the celestial equator. Its daily orbital motion is at the shallow angle of 5 degrees. So the advance in rise times starts off at 20 minutes later each night, rather than the average 50 minutes.
Spring Moon rising angle
How the Moon’s path near a spring full moon affects its rise time interval. Note the scale is not the same as the top image. Created using Cartes du Ciel (Sky Charts).

03/22/2021 – Ephemeris – The Moon and the week

March 22, 2021 Comments off

This is Bob Moler with Ephemeris for Monday, March 22nd. Today the Sun will be up for 12 hours and 16 minutes, setting at 7:58, and it will rise tomorrow at 7:39. The Moon, 1 day past first quarter, will set at 5:08 tomorrow morning.

The Moon tonight is about as far north in the sky as it gets this month, and being six days before full, and this full Moon is the first full moon of spring, that means Easter is less than two weeks away. Did you notice that a week is nearly equal to a quarter of the Moon’s orbit of the Earth? If you look at a calendar that has the Moon’s phases on it the phases are 7 or 8 days apart. It’s a good thing that no one decided to adjust the length of the week to accommodate the Moon’s phases. It’d be hard to plan your weekend. Our calendar is pretty screwed up as it is with months of 28, 29, 30 and 31 days all set by politicians… Roman politicians at that. I haven’t heard much about calendar reform lately, so I guess we’re stuck with it.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

I wrote the above before actually checking out the historical origin of the time period we call a week. It turns out that the week is really related to a quarter of a lunation or lunar month. The Babylonians fixed the fact that seven days is a bit less than a quarter of a lunation by adding a day or two to the last week of the month, so the next month’s new moon started on the same day of the week.

Here the new moon is defined as the first appearance of the Moon in the evening sky after disappearing from the morning sky. We currently define the new moon as the Moon’s solar conjunction. Its first appearance in the evening sky would generally be the next day, or rather evening. The numbering still works out. Our new moon is day zero of the lunar month.

03/16/2021 – Ephemeris – Tonight we’ll have exactly 12 hours of night

March 16, 2021 Comments off

This is Ephemeris for Tuesday, March 16th. Today the Sun will be up for 11 hours and 57 minutes, setting at 7:50, and it will rise tomorrow at 7:50. The Moon, 3 days past new, will set at 11:15 this evening.

Did you notice what I just said? Sunset’s at 7:50 pm and sunrise’s at 7:50 am. Tonight we’ll have exactly 12 hours of night. That’s what the word equinox means, equal night… But the vernal or spring equinox isn’t until Saturday when spring starts. By then night, including twilight, will be down to 11 hours, 50 minutes. The disparity comes down to modern versus older definitions. Equinox, being Latin is an older definition. The modern instant of sunrise and sunset occurs when the top of the Sun’s disc touches the sea horizon. Because our atmosphere bends light, and at its greatest when looking at the horizon, the Sun will have set already, geometrically, by the time the bottom edge of the Sun appears to touch that sea horizon. Making daylight a bit longer than you’d think.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

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

09/22/2020 – Ephemeris – Autumn starts this morning

September 22, 2020 Comments off

This is Bob Moler with Ephemeris for Tuesday, September 22nd. Today the Sun will be up for 12 hours and 8 minutes, setting at 7:39, and it will rise tomorrow at 7:31. The Moon, 1 day before first quarter, will set at 11:05 this evening.

Fall is about to a, well… fall upon us and in a few weeks so will the leaves. At 9:31 this morning (13:31 UTC*) 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 8 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 event times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

* UTC – Coordinated Universal Time. Greenwich Mean Time if you haven’t kept up. Zulu if you’re in the military.

Sun's motion near autumnal equinox

The Sun crossing the celestial equator in three steps: 9:31 am Sept 21, 22, and 23 2020. Created using Stellarium and GIMP.

Sun's path through the sky on the equinox

The Sun’s path through the sky on the equinox day from Traverse City, MI. 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. Credit: Bob Moler.

09/21/2020 – Ephemeris – This is the last full day of summer

September 21, 2020 Comments off

This is Ephemeris for Monday, September 21st. Today the Sun will be up for 12 hours and 11 minutes, setting at 7:41, and it will rise tomorrow at 7:30. The Moon, 2 days before first quarter, will set at 10:24 this evening.

Enjoy the last full day of summer. Summer will last until 9:31 am (05:31 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 for Earth’s northern hemisphere and spring will begin in the southern hemisphere. Shortly, for us, 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 or September equinox. The word equinox means equal night, implying the equality of day and night. Geometrically that’s true, but the Earth’s atmosphere and the definition of sunrise and sunset, prolong daylight by a few more 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 event times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

The Sun crossing the celestial equator in the sky moving southward in three steps: 9:31 am Sept 21, 22, and 23 2020. The vertical axis is declination, the exact match to latitude on the Earth. The horizontal line at 00°00′ is the celestial equator, a projection of the Earth’s equator on the sky. The diagonal line that the Sun appears to travel on is the ecliptic, the plane of the Earth’s orbit. Due to the tilt of the Earth’s axis it is inclined by 23.5° to the celestial equator. The horizontal values mark right ascension, the celestial analog of longitude. One hour equals 15°. Since the Earth rotates, the right ascension that is on one’s meridian, the north-south line passing through the zenith, is best kept track of by using a clock. A clock that runs 3 minutes 56 seconds fast a day. We call that a sidereal clock. Created using Cartes du Ciel (Sky Chart) and GIMP.

09/14/2020 – Ephemeris – Not exactly a mermaid

September 14, 2020 Comments off

This is Bob Moler with Ephemeris for Monday, September 14th. Today the Sun will be up for 12 hours and 33 minutes, setting at 7:54, and it will rise tomorrow at 7:22. The Moon, 3 days before new, will rise at 4:42 tomorrow morning.

Nearly 2000 years ago the southernmost of the constellations of the zodiac was Capricornus which is a 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 any more, Sagittarius, one constellation west past Jupiter and Saturn this year, has that honor today. Actually Capricornus does need the press. It’s 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-southeast at 10 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 mergoat.

The event times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

Capricornus finder animation

Capricornus finder animation for September 14, 2020 at 10 pm for western Michigan. Note that the Teapot of Sagittarius is pouring its contents on the southwestern horizon is to the right. Created using Stellarium and GIMP.

Important seasonal latitudes on the Earth. Source: worldatlas.com

 

 

 

09/10/2020 – Ephemeris – The bright star Deneb will pass overhead tonight, what does that mean?

September 10, 2020 Comments off

This is Ephemeris for Thursday, September 10th. Today the Sun will be up for 12 hours and 45 minutes, setting at 8:01, and it will rise tomorrow at 7:17. The Moon, at last quarter today, will rise at 12:29 tomorrow morning.

Around 11 pm tonight the bright star Deneb in Cygnus the Swan and the northernmost star of the Summer Triangle will be overhead, or just about at the zenith. Just as on the Earth we have a coordinate system of longitude and latitude for position east-west and north-south, we have the same for the celestial sphere the imaginary sphere of the heavens east-west is called right ascension and north-south is declination. I’m going to ignore right ascension’s relation to longitude. Declination directly relates to latitude in that a star the with the same declination as your latitude will pass directly overhead. Deneb’s Declination is about 44 degrees 20 minutes north. Check the GPS on your smart phone to see how close your latitude is to that.

The event times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

Celestial coordinates are actual projections of earthly coordinates. CNP = Celestial North Pole, CSP = Celestial South Pole are directly above the earthly poles. Same with Celestial Equator. Therefore a star or any celestial object passes overhead or at the zenith for locations with the same latitude on the Earth. That’s what the diurnal or daily circle is as the Earth rotates.

01/06/2020 – Ephemeris – The Earth was closest to the Sun in its orbit yesterday

January 6, 2020 Comments off

Ephemeris for Monday, January 6th. Today the Sun will be up for 8 hours and 57 minutes, setting at 5:17, and it will rise tomorrow at 8:19. The Moon, half way from first quarter to full, will set at 5:02 tomorrow morning.

Yesterday’s perihelion, or closest point of the Earth to the Sun of roughly 91.4 million miles (147 million km) is only 1.7% closer to the Sun than average. It doesn’t do much to make our winters warmer, but it does make winter the shortest season. That’s because the Earth travels faster when near the Sun than when it’s farther away. Winter lasts only 89 ½ days. The Earth’s aphelion, when it’s farthest from the Sun will be on the 4th of July, in summer, making that the longest season at 93 ½ days. Of course being this far north it feels like winter is longer than summer, but astronomically it’s the other way around. Being a leap year, with February having 29 days, spring will arrive a calendar day early on the 19th of March.

The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

Earth's orbit

The Earth’s orbit, somewhat exaggerated, showing perihelion and the seasons. Credit “Starts with a Bang” blog by Ethan Siegel.

Seasons for 2020

The Seasons for 2020 from data in Astronomical Tables of the Sun, Moon, and Planets Third Edition by Jean Meeus. Date and times are in TD, Dynamical Time. Subtract about 1 minutes to convert to Universal Time (UT).  Also subtract 5 hours for Eastern Standard Time and 4 hours for Eastern Daylight Time.

For and explanation of the Cross-Quarter Days column, check out my Ground Hog Day post last year:  https://bobmoler.wordpress.com/2019/02/02/02-02-2019-ephemeris-extra-groundhog-day-and-other-seasonal-days/

 

10/07/2019 – Ephemeris – Tides on and by the Moon

October 7, 2019 Comments off

Ephemeris for Monday, October 7th. Today the Sun will be up for 11 hours and 24 minutes, setting at 7:12, and it will rise tomorrow at 7:49. The Moon, 2 days past first quarter, will set at 2:21 tomorrow morning.

We always see the same face of the Moon turned toward the Earth. This does not mean that the Moon doesn’t rotate. It means that the Moon rotates on its axis in exactly the same time it takes to orbit the Earth. That is no coincidence. The effect of the Earth gravitation across the diameter of the Moon have essentially locked the Moon’s rotation to its revolution period. The crust of the far side of the Moon is thicker than the Earth facing side. The Moon is trying to do the same thing to the Earth. Its pull on the side of the Earth facing it is greater than the pull on the Earth’s opposite side. This stresses the Earth and raises tides in the ocean which actually slow the Earth’s rotation a tiny bit. As a consequence it pushes the Moon away by about 3.8 centimeters a year.

The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

Asymmetry of the crust of the Moon. Credit Lunar and Planetary Institute and Center for Lunar Science and Exploration.

For more information on this illustration:  https://www.lpi.usra.edu/exploration/training/illustrations/planetaryInteriors/ 4th illustration.

 

Categories: Concepts, Ephemeris Program Tags: , ,

09/13/2019 – Ephemeris – Harvest Moon tonight

September 13, 2019 Comments off

Ephemeris for Friday, September 13th. Today the Sun will be up for 12 hours and 38 minutes, setting at 7:57, and it will rise tomorrow at 7:20. The Moon, 1 day before full, will rise tonight 8:12.

Tomorrow is the Harvest Moon. In fact the instant of full moon is 12:33 a.m. tonight, so one could consider the Harvest Moon tonight. Funny thing though, this morning the Moon will reach apogee from the Earth, of 252 thousand miles (406 thousand km), making it the opposite of a super moon, a mini moon. I bet you wouldn’t notice if I didn’t tell you. The Harvest Moon is the name given to the nearest full moon to the autumnal equinox. It is a special time of the lunar cycle when the Moon rises much less than the 50 minutes later average each night. This appeared to extend twilight allowing farmers before the advent of electric lights extra time to gather in their crops each day.

The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

Harvest Moon effect

Harvest Moon effect from September 11 to the 15th, 2019. Note the shallowness of the ecliptic and Moon’s motion near sunset in this period. Click on the image to enlarge. Created using Cartes du Ciel (Sky Charts).