Archive for the ‘Concepts’ Category

03/15/2018 – Ephemeris – Mercury at greatest separation (elongation) from the Sun today

March 15, 2018 Comments off

Ephemeris for the Ides of March, Thursday, March 15th. The Sun will rise at 7:55. It’ll be up for 11 hours and 53 minutes, setting at 7:48. The Moon, 2 days before new, will rise at 7:45 tomorrow morning.

Today the planet Mercury will be at its greatest separation east from the Sun. It is called its greatest eastern elongation from the Sun. It is at an angle of 18.4 degrees from the Sun. It will be seen in the west about 8:15 for about an hour before it sets. It will be above right of the much brighter Venus. Mercury is probably at its best place to be observed than any time this year, with eastern elongation happening near the vernal equinox and is placed at a high angle above the Sun. The best morning appearance of Mercury will be its greatest western elongation on August 26th, almost a month short of the autumnal equinox, where it won’t be placed at as great an angle above the rising Sun.

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


Venus and Mercury

Venus and Mercury photographed last night at 8:20 EDT March 14, 2018. Did some tweaks to bring out Mercury in GIMP. Canon EOS Rebel T5, 75 mm, though reduced by 67%; f/4, 1/400 sec., ISO 1600. Click on image to enlarge. Click on image to enlarge. Credit Bob Moler.

Venus and Mercury positions tonight.

Stellarium’s showing of Venus and Mercury at 8:15 p.m. March 15. 2018.

Note in the above image, the steepness of the ecliptic (plane of the Earth’s orbit) is to the horizon in the spring.  Its angle to the celestial equator is 23.5°.  The angle the celestial equator makes with the horizon is your co-latitude (90° – your latitude).  At my location my latitude is 44.7°, so the celestial equator meets the horizon at 45.3°.  On the March  equinox the ecliptic, near where the planets hang out, reaches its most vertical at nearly 70°.  This makes planets, including Mercury appear higher in the sky near sunset, and as they set, moving parallel to the equator, will stay up their longest.

September equinox sunset

Celestial equator and ecliptic at the September equinox showing how low it appears. Created using Stellarium.

On the September  equinox the ecliptic, near where the planets hang out, reaches its most horizontal at near 22°.  This makes planets, including Mercury appear lower in the sky near sunset, and as they set, moving parallel to the equator, will set shortly after the Sun.  This September Venus happens to be approaching its inferior conjunction and is very close to the Earth.  This exaggerates its orbital inclination. in this case shows the planet a good deal south of the ecliptic.

Spring equinox sunrise

At the spring equinox close morning planets to the Sun will be hard to spot, being low to the horizon.

Ecliptic on the autumnal equinox

Celestial equator and ecliptic at the September equinox mornings showing how high it appears. Created using Stellarium.


The angles of the elongation of Mercury at the equinoxes

The angles of the elongation of Mercury at the equinoxes. Click on image to enlarge. Created using my LookingUp program.

Due to the eccentricity of Mercury’s orbit and its orientation with respect to the Earth’s positions at the equinoxes, observers on the southern hemisphere of the Earth get a better view of Mercury than us northerners.

(I created a similar diagram for posting yesterday, but found right before the scheduled posting time that it was incorrect in its orientation, so I redid it this morning.)


01/05/2018 – Ephemeris – Telescope Clinic tonight (Has been canceled due to weather)

January 5, 2018 1 comment

Ephemeris for Friday, January 5th. The Sun will rise at 8:19. It’ll be up for 8 hours and 56 minutes, setting at 5:16. The Moon, 3 days before last quarter, will rise at 9:56 this evening.

If you’ve received a telescope for Christmas and are having trouble setting it up, or have an unused one in a closet, basement or attic, bring them to Northwestern Michigan College’s Rogers Observatory tonight at 8 p.m. The Grand Traverse Astronomical Society will be holding their annual telescope clinic to help you understand and use your telescope. The clinic will extend through the period that will be set aside for a star party if it’s clear, to test the telescopes and show the owners how to use them. Like anything telescopes take some time to learn how to use them and find celestial objects. It took me 15 minutes to find Saturn with a telescope the first time I tried, and I knew where it was in the sky. The observatory is south of Traverse City on Birmley Road.

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


Link to my December 26 post on telescope basics to start you off:

Can’t make it tonight? Members are generally available at star parties at the observatory when time permits to help folks who bring their telescopes out. See for our current schedule of events for 2018.

Updated 7:02 p.m.

01/01/2018 – Ephemeris – The difference between the winter full moon and the summer one

January 1, 2018 Comments off

Happy New Year, this is Bob Moler with Ephemeris for New Years Day, Monday, January 1st. 2018. The Sun will rise at 8:19. It’ll be up for 8 hours and 53 minutes, setting at 5:13. The Moon, at full today, will rise at 5:11 this evening.

The exact time that the Moon will be fill, at least to the nearest minute is 9:24 tonight. Ever notice the placement of the full moon in the sky between winter and summer? The Full moon near the winter solstice moves very high at midnight, while the full moon near the summer solstice is seen quite low in the south. For the Moon to be full, it most be nearly opposite the Sun in the sky, so we see it fully illuminated as the Sun does. The Moon’s orbit is close to the Sun’s apparent path in the sky, the ecliptic, which is the projection of the Earth’s orbit of the Sun. So the Moon now is near where the Sun will be 6 months from now in late June, high in the sky. Next full moon we will see a lunar eclipse.

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

The winter full moon

The winter full moon at its highest. Created using Stellarium.

Summer full moon

The summer full moon at its lowest. Created using Stellarium.

12/26/2017 – Ephemeris – Help for that Christmas telescope

December 26, 2017 1 comment

Ephemeris for Tuesday, December 26th. The Sun will rise at 8:18. It’ll be up for 8 hours and 49 minutes, setting at 5:08. The Moon, at first quarter today, will set at 1:34 tomorrow morning.

Get a telescope for Christmas? Or is there one lurking in a closet or attic? Tonight’s a good time to get it out, if it’s clear, because there’s a first quarter Moon out. I consider the Moon round the first quarter to provide the finest viewing.  To get started, most astronomical telescopes have a small finder scope attached. Daytime is the best time to align them on a distant object. To find anything, even the Moon, use the lowest power eyepiece. In most scopes that’s a 20 to 25 millimeter eyepiece. Eyepieces with lower numbers are higher power. Most amateur astronomers use their lowest magnification 90% of the time. The bright planets right now are in the morning sky, and only Jupiter is worth looking at. It’s easy to find because it’s the brightest star-like object, and in the southeast before 7:30 a.m. To help you further, on January 5th, at Northwestern Michigan College’s Rogers Observatory the Grand Traverse Astronomical Society will host its annual Telescope Clinic for telescopes new and old.

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


Here’s a handout we make available at these telescope clinics:

Telescope Basics

Telescope Types

                           Telescope Types

There are two basic telescope types: The refractor or refracting telescope and reflector or reflecting telescope. The reflecting type shown is a Newtonian telescope, is the simplest and most inexpensive reflector. The Catadioptric (Mirror-Lens) telescope uses a corrector plate in front of the telescope. The one shown is a type called a Maksutov-Cassegrain. The more popular type is the Schmidt-Cassegrain which uses a thin, nearly flat corrector plate. The Cassegrain design uses a convex secondary mirror the sends the light back through a hole in the Primary mirror (O) to an eyepiece.

Refracting telescopes get expensive in a hurry as the diameter of the objective (O) lens increases due to the requirements of at least 4 lens surfaces of the at least two lenses that make it up. The reason for it is to correct for color fringes that would result around bright objects seen through it (chromatic aberration), and the optical quality of the glass required. Reflectors primary mirror have a single surface and the glass simply supports it. The corrector plates of the catadioptric telescopes don’t create chromatic aberration because they don’t bend light much. Telescopes are rated by the diameter of their objectives (O). One could purchase an 11 inch Newtonian telescope for less than $1,000, An 11” Schmidt-Cassegrain for $2,500, or an 11” refractor for the cost of a Lexus.

The reason astronomers go for wider telescopes (greater aperture) is two-fold: To gather more light to better see faint objects, and to increase resolving power, the ability of the telescope to see fine detail and be able to use higher magnification. We’ll see the rules when we talk about eyepieces.

Telescope Mounts

Telescope MountsThere are four basic mounts. Equatorial mounts have to be aligned to the earth’s axis in order to work properly to follow objects in the sky. Alt-Azimuth mounts are the simplest and easiest to set up, but all but the most sophisticated cannot be made to track objects in the sky as the Earth rotates. A relatively new addition to mounts is the computerized “Go To” feature allows the telescope to find objects itself when the mount is properly aligned to the sky. Telescopes with Dobsonian mounts have the largest aperture for the buck. Cheap telescopes tend to have cheap mounts that are hard to use and wobbly, especially the ones with German equatorial mounts. An alt-azimuth mount would be steadier and a whole lot easier to use in this case.

Finder Telescopes

The telescope eyepiece covers so little area of the sky to make finding anything virtually impossible. So all telescopes have small finder scopes attached of 6 to 10 power, or 1 power devices that put a finder circle or red dot on the sky when you look through them. A newer finder idea is a mount for a green laser that projects a beam in the atmosphere toward the object to be located. The author prefers a finder with an aperture of at least 50mm to be able to see most of the dim objects he’s looking for. In the main telescope, use the lowest power eyepiece because it has the widest field of view.


Magnifying power or magnification is not a telescope property. The eyepiece is essentially a magnifying glass to view the real image that the objective lens or mirror produces at the focal plane (F) in the telescope type diagram on the first page. The focal length of the objective lens or mirror or the effective focal length of the mirrors of the catadioptric telescope divided by the focal length of the eyepiece gives the magnification of that particular combination of telescope and eyepiece. The focal length of the eyepiece is marked on the eyepiece. The telescope focal length may or may not be stamped or marked on the telescope, if not, check the owner’s manual for that quantity.

A telescope will generally come with one or two eyepieces, The lowest power eyepiece will generally be a 25mm eyepiece of some kind. Eyepieces come in 2 standard barrel sizes, 1 ¼ inch and 2 inch. There are some old telescopes that only accept sub 1 inch eyepieces. You may have to hunt to see if any of those size eyepieces are still around. The cheaper the telescope the crummier the eyepiece. Decent eyepieces start at around $35 and go up from there. The best way to tell which eyepiece fits your needs is to ask an astronomer what eyepiece he or she is using at a star party.

About magnification. The highest usable magnification in a telescope is calculated as the aperture in millimeters times 2.4 or aperture in inches times 60. After that the image becomes fuzzy and dim. It’s due to the wave nature of light. I halve those values in his experience. I’d rather have small crisp images than big fuzzy ones devoid of contrast.

A handy accessory to have is a Barlow lens, a negative lens in a tube, that the eyepiece is slipped in before inserting the pair in the eyepiece holder. It will double the power of the eyepiece. So with two eyepieces and a Barlow four separate magnifications are available. The author would rather us a lower power eyepiece with a Barlow than a high power eyepiece of the same power. In that same vein a good low power wide-angle eyepiece is generally the first extra eyepiece astronomers purchase. More expensive ones can be like viewing the universe in IMAX. Here is a truism: Amateur astronomers use their telescope’s lowest power 90% of the time.

Solar filters that fit over the front of the telescope and finder is a fine addition to any telescope and allow viewing of our star close up. Some old telescopes have a solar filter that fits in an eyepiece. For your visual health take the filter and beat it to death with a hammer, and throw it away. There are also filters that can filter out some of the light pollution for dim nebulae. There are filters also to bring out detail in planets.

Above all, have fun! If you have any questions ask that friendly amateur astronomer over there, standing by his or her telescope at the next star party.

12/21/2017 – Ephemeris – Winter begins this morning

December 21, 2017 1 comment

Ephemeris for Thursday, December 21st. The Sun will rise at 8:16. It’ll be up for 8 hours and 48 minutes, setting at 5:05. The Moon, 3 days past new, will set at 8:23 this evening.

The thermometer and snowfall tell us that winter ought to be here. Well it will be at 11:29 this morning. At that point the Sun will be directly over the Tropic of Capricorn at 23 ½ degrees south latitude. Odd name because 2,000 years ago the Sun was in indeed entering Capricornus. Now it’s entering in Sagittarius, right above the spout of the teapot asterism we know so well in summer. From then on the Sun will be climbing up the sky each noon until June 21st next year when summer will start. To which I say Go Sun Go! The Sun will barely make it to 22 degrees above the southern horizon at local noon in Interlochen and be out for only 8 hours and 48 minutes. If it stayed that low all year we’d be in a deep freeze possibly colder than Antarctica.

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


The Earth near the December solsitce

DSCOVR satellite’s Earth Polychromatic Camera image of the Earth at 18:09 UTC (1:09 p.m.) December 19, 2017. We’re way up at the top just under the clouds at the top. It was actually partly cloudy that day. The DSCOVR satellite was in a halo orbit about the Earth-Sun L1 point, 934,498 miles (1,503,929 km) toward the Sun from Earth.

The Sun at the solstice

The Sun is shown against the celestial sphere at the moment of the December solstice. Saturn will be in conjunction later in the day, heading to the right along the ecliptic, which is the path of the Sun. Venus will pass the Sun, heading to the left on January 9th. The grid lines are right ascension in hours (h) and declination in degrees. The solstice occurs when the Sun crosses the 18 hour line. Note the Teapot asterism that is part of Sagittarius. Created using Stellarium.

12/19/2017 – Ephemeris – Were celestial events in 7 BC interpreted as the Star of Bethlehem?

December 19, 2017 1 comment

Ephemeris for Tuesday, December 19th. The Sun will rise at 8:15. It’ll be up for 8 hours and 49 minutes, setting at 5:04. The Moon, 1 day past new, will set at 6:34 this evening.

This year we have no bright evening Christmas star. But what about the one described in the Bible, in the gospel of Luke? We will look today at the first of two events that may have been recorded as the Star of Bethlehem. In 7 BC there was a rare event over 6 months when three times the planet Jupiter passed Saturn against the stars of the constellation Pisces. Could the Persian astrologer priests called Magi, have read into the event enough significance to start the journey to Jerusalem in search of the new born King of the Jews? It was the scribe’s readings that then sent them to Bethlehem. It is thought that Pisces was associated with the Hebrews, Jupiter with the Messiah or world ruler, and Saturn with the peoples of Palestine.

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


Triple conjunction

The Jupiter-Saturn triple conjunction of 7 BC. Click on the image to enlarge. Created using Cartes du Ciel.

The animation above shows the planets at 5 day intervals in 7 and a bit in 6 BC.  The normal or posigrade motion is toward the east or left.  When an outer planet approaches its opposition from the Sun, it seems to slow and reverse direction and head westward for a time, which is retrograde motion.  After a time it reverses and heads back eastward again.  That’s why the planets seem to see-saw back and forth.  Jupiter passes Saturn every 20 years.  The next time will be in 2020.  On my Wednesday planet charts Jupiter is seen sneaking up on Saturn week by week.  In order to have three conjunctions instead of one, the planets need to be near opposition from the Sun when they pass.  The last time that happened was in 1980, but it wasn’t against Pisces.  Triple conjunctions against a particular zodiacal constellation are much rarer.

The dates for key events in the animation above are:

  • First Conjunction May 29, 7 BC
  • Planets are Stationary July 6, and begin retrograde motion
  • Second Conjunction October 11
  • Planets are Stationary November 1 and resume posigrade motion
  • Third Conjunction on December 8
  • Mars joins February 26, 6 BC

Retrograde motion was difficult to explain when one thinks that the Earth is the center of the Universe and stationary, while the planets supposedly orbited in perfect uniform circular motion around it.  It’s easy when the Earth is a moving planet like the rest.  When an outer (superior) planet is at opposition from the Sun.  The Earth is between the Sun and planet and moving faster.  So we’re passing the outer planet.  When you are in a car passing another, the car being passed seems to move backward with respect to your car.  Since the planets orbits are like a race track, we get to lap the outer planets at regular intervals.

12/14/2017 – Ephemeris – The Moon wanders over to Jupiter this morning

December 14, 2017 1 comment

Dec 14. This is Bob Moler with Ephemeris for Thursday, December 14th. The Sun will rise at 8:11. It’ll be up for 8 hours and 50 minutes, setting at 5:02. The Moon, half way from last quarter to new, will rise at 5:26 tomorrow morning.

This morning the planet Jupiter will appear right below the crescent Moon. Jupiter is hard to miss, even without the Moon to point it out. It is with the rare exception of Mars when being its closest to the Earth the second brightest of the planets, after Venus. Speaking of Mars, which is to the upper right of Jupiter and has a reddish hue, if you’re going to send anything to Mars, next spring is the time to do it. Flight times to Mars are 6 to 7 months. The midpoint of the flight is when Mars is closest to the Earth, which next year is July 31st. NASA’s Insight Lander, grounded in 2016 due to an instrument failure has to wait 26 months for the next launch opportunity in May of next year.

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


The Moon, Jupiter, Mars

The Moon, Jupiter and Mars this morning, December 14, 2017. Earth shine should be visible as shown, though not as prominent. Created using Stellarium.

Hohmann orbit to Mars

A Hohmann lowest energy transfer orbit to Mars. This diagram is for the Mars Exploration Rovers Spirit and Opportunity launched in 2003, arrived in 2004. Solid planets, Spirit launch and arrival. Ghost planets, Opportunity launch and arrival. Credit NASA/JPL.

What’s a Hohmann transfer orbit?  NASA explains.