The Meridian. Accessed by Jim Johnson on January 31, 2015.The meridian is a special line in the sky that is related to an observer’s location. It is defined by three points: 1) the north point on the horizon, 2) the zenith, and 3) the south point on the horizon. By extending an arm horizontally and pointing north, then swinging the arm upward until it is pointing straight up overhead, and continuing in that direction, which is now a downward motion, until reaching the south point on the horizon, an observer has traced the meridian.

The meridian remains stationary while the celestial sphere appears to rotate past it as the earth rotates about its axis. All objects that we observe on the celestial sphere appear to move perpendicular to the meridian. When an object on the same declination as the observer’s latitude rises from the east and ascends to the meridian, it has reached its zenith. From then on, the object descends toward the western horizon. Objects that are at a greater or lesser declination than the observer’s latitude have also reached their highest point when crossing the meridian, but will cross the meridian either north or south of the zenith.

The knowing what time an object reaches the meridian is important for an observer wanting to get the very best possible view of an object. The view toward the zenith is the clearest possible view, because the observer is looking through the thinnest possible cross section of the earth’s atmosphere. The view toward the horizon, however, is through the thickest possible cross section of the atmosphere, and this is where atmospheric haze will degrade viewing conditions the most. No matter the object’s declination, the time that it crosses the meridian is its closest approach to the zenith, and the very best possible time to observe.

Image Credit:
– The Meridian. Accessed by Jim Johnson on January 31, 2015.

© James R. Johnson, 2015.


Horizon and Coordinate System, Accessed by Jim Johnson on January 19, 2015.Simply stated, the zenith point in the sky is the point that is straight up above a specified location on the earth’s surface. Its location can be described more precisely by imagining a line originating at the center of the earth, extending up to the surface at the observer’s feet and exiting the top of the observer’s head. The point where this line touches the celestial sphere is the zenith point. It can also be defined as a 90° angle upward from any, or from every, point on the horizon.

Celestial Sphere, ASTR 1230 (Majweski) Lecture Notes. Accessed by Jim Johnson on January 19, 2015.The zenith’s location is relative to the observer’s location. So long as the observer remains in a fixed location, the zenith remains fixed, and stars of the same declination as the observer’s latitude will appear to move through the zenith as the earth rotates. If the observer moves from that location by walking north, for instance, then the zenith moves north along with the observer.
Zenith. Accessed by Jim Johnson on February 1, 2015.As previously alluded, the declination of the zenith is the same as the observer’s latitude. If an observer is standing of the terrestrial equator, then the observer’s zenith is on the celestial equator, which turns overhead as the celestial sphere appears to turn as a result of the earth’s rotation about its axis. Also, if an observer is standing on the terrestrial north pole, then the zenith will be at the celestial north pole, and all stars will appear to circle the zenith as the earth rotates.

Image Credits:
– Horizon and Coordinate System. Accessed by Jim Johnson on January 19, 2015.
Celestial Sphere. ASTR 1230 (Majweski) Lecture Notes. Accessed by Jim Johnson on January 19, 2015.
Zenith. /02.motion_stars_sun/. Accessed by Jim Johnson on February 1, 2015.

© James R. Johnson, 2015.

About Scope Out Next Month

Scope Out is a monthly publication for casual stargazers in the mid-northern latitudes. It highlights the constellations that are highest in the sky and are therefore in the best viewing position at nightfall, describes the locations of the solar system’s planets, marks the date of the Moons phases and conjunctions with the planets, and it describes the interesting astronomical events that occur during the month.

Understanding a Star Chart

Welcome to the second in my series of guides to observing the night sky. This article is written under the Constellations Project, and it builds on How to begin Observing the Night Sky by describing the celestial coordinate system and other details that appear on the sky maps presented in each month’s edition of Scope Out Next Month. As an aid to understanding this material, I have referenced and inserted thumbnail images of graphics that can be clicked for a larger view.

CelSphThere are two related spherical representations to consider in order to learn how to read a a star chart – a globe and the celestial sphere. Think of the Earth as the terrestrial sphere situated at the center of the celestial sphere. An observer looking outward from the Earth is gazing upon the celestial sphere. A globe is a spherical representation of the Earth’s surface that shares several common attributes with the celestial sphere.  For this reason, a brief review of a globe is a good starting point for understanding a star chart. Refresh your understanding of a conceptual celestial sphere, if needed, by reading How to begin Observing the Night Sky. 

globeWe can see on a globe that the land areas are apportioned among countries with borders (lines) drawn to demarcate each country’s political, or man-made boundaries. Within the countries are cities. The cities are generally regarded as fixed points that may from time to find themselves in one country or another as political boundaries are redrawn. Highways connecting the cities can be found on maps representing smaller portions of the Earth’s surface.

FebruaryConstellations are the man-made corollaries to the countries, cities and highways that we find on globes and maps representing the Earth surface. Referring to the accompanying star chart, which is made specifically for Ashton, MD at 8pm EDT on March 15, 2015, find Orion located  below and to the right of the zenith (the red circled “z“). There are two ways to think of constellations. First, and foremost, a constellation is a grouping of stars (cities). In this chart Orion appears as a stick figure torso with a belt of three stars and what is usually depicted as a shield and raised club in the chart. The interconnecting blue lines (highways)  aid in identifying the spatial relationships between the constellation’s stars by guiding our eye from one star to the next. Secondarily, a constellation is an area of the sky (country), which is delineated by the purple border. If the Sun, Moon, a planet, or some other celestial object happens to appear within a constellation’s borders, it is said to be located in that constellation.

Now to take on the two coordinate systems. Both spheres have horizontal and vertical circles, sometimes called  lines, that represent north-south and east-west spatial relationships. The intersection of two lines is a coordinate that represents a point on the surface of the sphere. For instance, if I know that Ashton, Maryland is located at N39.14° and W77.01°, I can locate Ashton on the globe by finding the intersection of the N39.14° and W77.01° lines on a globe or on a map. Similarly, the intersection on the celestial sphere of the lines representing declination +16.5°, and 4 hours, 36 minutes right ascension is where the star Aldebaran is found in the constellation of Taurus. Aldebaran, a bright star, is not named on the map above, but it can be found at Dec +16.5 degrees, Ra 4:36.

601lat_longThe horizontal lines, or circles, on the globe are called parallels of latitude. The equator, or the largest circle half way between the two points were the globe is attached to its stand, is a special parallel that is designated as the zero degree latitude. One could travel around the world while remaining on the equator and never be at a latitude of more than 0° north or south. More circles called parallels of latitude can be found in 10 to 15 degree increments as one moves north or south away from the equator and toward the two attachment points. Note that these circles do not cross one another (they’re parallel), and each circle is smaller than the one before as one moves north or south away from the equator.  Latitude is a measure of the angular separation in degrees away from the equator. Imagine yourself sitting at the center of the Earth, with the top of your head pointing toward the north pole while your eyes gaze outward at the equator, the zero-degree latitude. As you being to tilt your head back to look up toward the north pole, the angle between the equator and the point at which you are looking increases, and is the measure of the point’s latitude. If you look upward far enough to see Ashton, Maryland, then the angle, and the latitude, is N39.14°.

celestial_sphere_decThe celestial sphere’s corollary to latitude is called declination, and declination is very closely related to latitude. In fact, both are measured in degrees. If one is standing on the equator, then the celestial equator, or 0 degrees declination, is directly overhead at the zenith. Points north or south of the celestial equator are indicated in positive numbers north of the celestial equator, and negative numbers south of the celestial equator. When standing in Ashton, Maryland for instance, the declination at the zenith is +39.14°. This can be observed on the sky chart prepared for Ashton Maryland that was previously examined. Note that the zenith falls just south of the declination line that represents +40°.

The poles on the both the Earth’s sphere and the celestial sphere are two special latitude and declination points. They are defined as the point where the extension of the Earth’s axis beyond its surface intersects the celestial sphere. I referred to the poles as points (as opposed to lines) because the Earth’s celestial poles are at N90° and S90°. There is no east/west component, because the only direction away from, say the north pole, is south. When standing on one of the Earth’s two poles, the corresponding north or south celestial pole will be at the zenith. These points have a single coordinate of +90° or -90° for the north and south celestial poles, respectively.

601lat_longLongitude, the east-west measure of location in degrees, is represented by the vertical lines on the globe. Note that these lines converge on, and meet at both the north and south poles. The prime meridian, which is the longitude that runs north-south through Greenwich, England, is designated as the zero degree of longitude. Sitting back at the center of the Earth again, turning your head away from the prime meridian increases the longitudinal angle increases. Looking west from the prime meridian to Ashton, Maryland’s location, the angle will be W77.01° from the prime meridian. Longitude can be written as either E or W of the prime meridian, usually no more than 180°. The longitude on the opposite side of the Earth from the prime meridian is correctly written as either W180°, or E180°.

Dec_and_RightAscensionThere is a corollary to longitude on the celestial sphere, but it is not as closely related as latitude and declination are. The vertical, or north-south, lines on the celestial sphere are known as right ascension, and are measured not in degrees, minutes and seconds, but in hours, minutes and seconds. This difference in the units of measure accounts for the changing relationship between the “fixed” surface of the Earth  and the  celestial sphere that “rotates” across the sky. There are twenty-four hours of right ascension on the celestial sphere, which is related to the length of a day on the surface of the Earth. Two different hour lines of right ascension are one hour, or 15° apart. Fifteen degrees is exactly how much the celestial sphere appears to rotate in one hour. Twenty-four right ascension hours spaced 15° apart represent the 360° circle around the celestial sphere.

bt2lf0209_aThere is a special right ascension line at a given location on the Earth’s called the meridian. The meridian is defined as an arc that starts at the north point on the horizon, extends up toward through the zenith, and back down to the the south point on the horizon. The right ascension of the celestial sphere that is on the meridian is known as local sidereal time. If the 2nd hour of right ascension on the celestial sphere happens to be on the meridian at a given time, and say an hour passes, then the 3rd hour of right ascension will be on the meridian. Let another hour and a half pass, and the right ascension at the meridian will be 4hours 30 minutes, or 4h30m.

The material covered in this post is really all that is needed to understand the coordinate systems and constellations on a star chart. I have alluded to the dynamic nature of a celestial sphere that appears to turn over our heads, and the next in this series of posts will expand upon how this sphere turns with respect to the Earth’s diurnal (day and night) rotation, and with respect to the Earth’s annual orbit about the Sun.

Questions and discussion are always welcome.

© James R. Johnson, 2015.

Image Credits:
– Celestial Sphere, ASTR 1230 (Majweski) Lecutre Notes. Accessed by Jim Johnson on January 19, 2015.
– Declination on the Celestial Sphere. Accessed by Jim Johnson on January 27, 2015.
The Globe. Accessed by Jim Johnson on January 27, 2015.
– Latitude and Longitude. Accessed by Jim Johnson on January 27, 2015.
Celestial Sphere with Declination and Right Ascension, Steven Schimmerich, Accessed by Jim Johnson on January 27, 2015.

February 2015


While most of us are ready for spring, the seemingly endless winter drags on. For the brave among us who can endure the cold nights, this is a great time for stargazing – it still gets dark early, the sky is generally clear more often, and some of the prettiest constellations are well placed for viewing right after sunset. At the very least, look up at the sky and take note of what you observe if you happen to be outside after dark in the evenings or before sunrise in the mornings.

About Scope Out      How to begin Observing the Night Sky


The sky map below represents the sky as it will appear in mid-February at the end of astronomical twilight,  the arrival of complete darkness, at 7:15pm EST. The Scope Out monthly focus will be on the constellations that are  just to either side of the meridian, which is halfway between 4th (4h) and 6th (6h) hours of right ascension line in the February sky map. For a primer on how to use this sky map, please read How to begin Observing the Night Sky.

Scope Out divides the celestial sphere into three zones to aid in finding constellations:

1. Circumpolar Constellations:  Find Lynx, and Perseus in the northern sky above Polaris.

2. Northern Constellations:  Orion is at its highest nightfall ascension of the year, a position from which it steals the night time show.  Find February’s remaining northern constellations, Taurus, Gemini, Auriga, and Canis Minor, near the zenith. Be sure to look for the Pleiades cluster near Taurus, and the binocular view is breathtaking.

3. Southern Constellations: The best-placed constellations in February are Monocerous, Canis Major, and Lepus. Noteworthy among Canis Major’s stars is Sirius, the dog star, which is the brightest star in all of the night sky.



Colors of the planets. This picture is not to scale. Image from NASA’s Planetary Photogrounal at

Mercury is no longer visible in the western sky. By mid-month, this fleet-of-foot planet will have moved past the Sun, and will appear as a morning “star” by month’s end. Look for it low on the eastern horizon about an hour before sunrise on February 16th and 17th. Venus and Mars will share the western horizon, and can be seen about an hour after sunset. Venus will appear a little higher above the horizon each evening, and Mars will appear a little lower each evening. They will be at their closest approach, or in conjunction, as they pass one another on February 21st.  Jupiter rises in the east just after sunset, and is at its closest to approach to Earth, or at l be in opposition, on February 6th. Telescope observers will not want to miss this! Jupiter’s four Galilean moons and its dark equatorial bands can be seen with the aid of a modest telescope. Saturn is presently a morning object, rising in the east at about 2:30am at the beginning of the month, and rising just a little earlier each morning. Uranus is located near Venus around month’s end, and can be found with binoculars or a modest telescope. Neptune is near Venus at the beginning of the month, but this dim planet will be very difficult to observe in the twilight.


moon_phases_small_full February 3
Full Moon
February 3-4
Conjunction with Jupiter (evening)
moon_phases_small_lastqtr February 11
Last Quarter
February 12-13
Conjunction with Saturn (morning)
February 17
Conjunction with Mercury (morning)
moon_phases_small_new February 18
New Moon
February 21
Conjunction with Venus and Mars (evening)
moon_phases_small_firstqtr February 25
First Quarter

© James R. Johnson, 2015

How to begin Observing the Night Sky

Observing the night sky can be quite easy under the right conditions. In its simplest form, all that is required is that one be outside after dusk on a cloudless evening, and just look up. A moonless sky away from the city lights will reveal a sky full of stars while a city observer under a full Moon will see just a few stars. This simplest form of observing the night sky is all that some stargazers will ever want to know how to do, while other stargazers will want to make some sense of the sky, and perhaps even be able to find or identify constellations. Knowing where to start can be intimidating for some, so my sincere hope is that this article enables an enhanced observation experience by gaining an understanding of how a star chart is used to find and identify constellations and other celestial objects.

The Sky – A Stargazer’s Working Definition

stars-in-the-sky-swedenA star chart will be of little use until an observer understands the sky that the chart represents, so developing a basic understanding of the sky is important. Defining the sky is rather simple: walk out into the middle of a large, open field, or just imagine yourself standing there. Lift one arm until it is horizontal, and point in any direction at the “line” where the sky meets the earth. This is a point (see what I just did there?) on the horizon. Keeping your arm extended horizontally, turn about in a complete circle, and the circle traced out by the extended finger has traced out all of the points that make up the horizon, which we will simply refer to as the horizon. All that is below the horizon is earth, and all above is sky.

Next one must determine which direction on the horizon is north to permit proper alignment of the star chart. Please read Cardinal Directions for Casual Observers for some “pointers” on this topic.

bt2lf0209_aZenith is another point in the sky that is an aid in locating constellations. To locate this point, imagine a line drawn from the center of the Earth, up to its surface at point at which you are standing and through your body, and extending out of the top of your head. The point at which this line touches the celestial sphere above you is the zenith. No matter where on the surface one might be standing, or what time or what day it is, this simple rule defines the zenith. Zenith is located at the center (not coincidentally) of the sky chart and is indicated by the circled “Z.”

All of the stars and other objects that might be seen in the sky are at varying distances from us. Since even the nearest of these objects are so far away, our mind perceives them as all being at the same indeterminable distance away. For this reason and for the purpose of this amateur stargazers, the sky and all that it holds can be thought of as the celestial sphere – much like the inside of a planetarium dome. Stars are fixed points upon this sphere that all move together across the sky. Imagine the Earth at the center of this sphere, and that the sphere surrounds the entire Earth.

CelSphIn reality, the Earth spins on its axis beneath the celestial sphere. The point on the Earth’s surface from which we happen to be observing circles the Earth’s once each day, and we observe the celestial objects passing through our field of view, or the sky, from east to west. Our perception is that the Earth is fixed, and the celestial sphere turns above us. For casual observational purposes, and for ease of explanation, the remainder of this discussion will assume the perceived motions.

The Star Chart

At the beginner level, a chart prepared for the observers location and time is required. Below is a representative chart of the chart that I post in monthly editions of Scope Out Next Month. This particular chart represents the celestial sphere over Ashton, Maryland at nightfall on March 15, 2015. For casual observational purposes, this chart is good at nightfall for the entire month of March 2015, and for mid-latitude northern locations. Be sure to grab an up-to-date chart from the current month’s Scope Out. Click to expand the image if you would like, and take a few moments to look it over before reading on.

Sky chart for March 15, 2015 at nightfall over Ashton, Maryland. Jim Johnson (December 27, 2014).

The outside edge of the chart is a circle, which represents the circular horizon that we traced out early on. Note the N, S, E, and W points at the edge of the horizon. Notice that the east and west points are on the sky chart are opposite of the east and west points that we are accustomed to seeing on a map. Why? Because instead of looking down on a map, we are now looking up at the sky!

Observing the Night Sky with a Sky Chart

star_gazing_coupleFirst, grab an up-to-date sky chart from the Scope Out archive, get comfortable and get aligned. Comfort for star gazing would be lying flat out on the ground, perhaps on a padded surface, or reclined in suitable lawn furniture. Dressing for the current weather conditions is important as well. Being aligned means that the top of one’s head is pointing north. From a reclined position, east will be to the observers left, west to the right, and south toward the feet. Now hold the chart up against the sky, and identify the constellation that you wish to find. Using the example sky chart at Ashton, Maryland at nightfall on March 15th, 2015 as an example, one might locate the zenith, and look south and slightly west to find Orion. Looking further south, toward the feet, I could find Canis Major. I could find Leo by looking to my right, halfway down toward the horizon, and I could look up toward the top of my head to find the northern constellations, like Ursa Minor, or the Little Dipper.

That’s all there is to observing the night sky with a sky chart prepared for the date, time, and location of the observer. Understanding a Star Chart, which builds upon the concepts presented here, is the next logical progression to constructing a more complete understanding of the night sky.

How to begin Observing the Night Sky, in my mind, is probably the most important article that I have posted to my AstroLog. I would appreciate your feedback to make easier to follow or understand, and your questions are always welcome. Drop me a line at

Image Credits:
Horizon and Coordinate System, Accessed by Jim Johnson on January 19, 2015.
Celestial Sphere, ASTR 1230 (Majweski) Lecutre Notes. Accessed by Jim Johnson on January 19, 2015.- — – Stargazing Couple, Accessed by Jim Johnson on January 19, 2015.

© James R. Johnson, 2015.

Last Chance to View Comet Lovejoy

The Complex Ion Tail of Comet Lovejoy by V. Popov & E. Ivanov  from Astronomy Photo of the Day, 01/21/2015 (
The Complex Ion Tail of Comet Lovejoy by V. Popov & E. Ivanov from Astronomy Photo of the Day, 01/21/2015 (

Comet Lovejoy has been the center of attention in the night sky all month. It was difficult to observe earlier this month because it was low on the south eastern horizon right after dark, and the presence of a bright Moon removed any possibility of viewing it with the unaided eye. Beginning around January 8th through now has been the best time to observe the comet, because the Moon was no longer above the horizon in the early evening, the comet was near its peak brightness, and it was placed higher in the sky. As the first quarter Moon approaches on January 26, however, Lovejoy will be increasingly difficult to observe. Although no longer a naked eye object, any clear evening over the next few days will be the last best opportunity find it with binoculars this month. By the time the Moon’s phase is favorable again will be around mid-February, and Comet Lovejoy will have dimmed considerably.

Presently, Comet Lovejoy is nearly directly overhead at the zenith in the early evening. Using the star chart in Sky & Telescope magazine’s article Where to See Comet Lovejoy Tonight, it is easy to find Lovejoy by first locating Orion, tracing a line toward Taurus and then through and to the west of the Pleiades. My observation reports on Comet Lovejoy (C/2014 Q2) describe what you can expect to observe. As usual, I want to know about your observations, so please drop me a line.

Cardinal Directions for Casual Stargazers

Compass Rose, Brosen, April 13, 2006. Retrieved from Wikipedia by Jim Johnson on January 19, 2015.
Compass Rose, Brosen, April 13, 2006. Retrieved from Wikipedia by Jim Johnson on January 19, 2015.

Being able to identify the four cardinal directions – north, south, east, and west – is necessary so that stargazers can correctly orient a star chart to permit a desired constellation might be found. Close is good enough for our purpose, and once any one of the cardinal directions is determined, the other three are easily derived by imagining our body as a compass rose. I will describe a compass rose stance and how to use it, and I will present three easy methods to determine at least one cardinal direction. Many more methods can be found by a Web search on “finding cardinal directions without a compass.”

Assume a compass rose stance by standing up, and extending the arms up and straight out from the sides, and horizontal to the ground. Assume that you already know which way is north and that you are facing in that direction. Looking at the compass rose above, we can see that the face is the north point of the rose, the right hand is east, the left is west, and the south point is the back of the head. If we can determine any one of the four cardinal directions, and turn our self until the correct point of the compass rose stance is aligned in that direction, then we can determine the other three cardinal directions off of the the other three points of our compass rose stance.

The easiest method to find one cardinal direction is based on the Sun, and can be used if one knows where the Sun rises and sets. The Sun rises in the east, and it sets in the west. That orange glow on the horizon before sunrise and after sunset also indicates which directions are east and west, respectively. To illustrate, if I am observing in the early evening and can still see the sunset orange glow is brightest at one point on the horizon, I have determined which direction is west. Assuming my compass rose stance with my left hand pointing toward the west, I know that north is straight ahead, east is to my left, and south is behind me.

A compass is just as easy as working of off the sunrise and sunset points if one is available. Since the red arrow will always point north, an observer can face in direction of the red arrow and derive the other three directions by extending the hands toward the horizon. The left hand points west and the right hand points east. South, of course, is behind the observer.

Once north has been determined, it is easier to find Polaris, which can be used in the middle of the night from any location. Read Polaris, The North Star to learn how to find this star and use it to determine which direction is north. I recommend that any serious stargazer invest the time in learning this method.

One of these processes or a similar process must be used to determine the cardinal directions the first time that a stargazer observes from a new location. If a stargazer is to make repeated observations from the same point, then the cardinal directions need only be determined once and committed to memory or noted in an observing log. For instance if an observer remembers that a distinct tree on the distant horizon that is just a little left of North, then using that tree as a future reference, the observer readily knows which direction is north on subsequent visits to that spot.

Knowing the cardinal directions is an inherent part of observing the sky. Learning how to determine these directions is quite easy, and becomes second nature with just a little practice, and after a while the compass rose stance will no longer be needed.

© James R. Johnson, 2015.

Conjunction of Venus and Mercury – January 9, 2015

Conjunction of Venus and Mercury. January 9, 2015. James R. Johnson
Conjunction of Venus and Mercury. January 9, 2015. James R. Johnson

Venus (upper right) and Mercury (lower left) are in close conjunction on the date of this post. The accompanying image was taken on the day before conjunction, and just before the two planets followed the setting Sun below the western horizon. I happened to be at Howard Astronomical League’s Alpha Ridge dark site for an observing session, and had forgotten about the conjunction. Once I spotted the pair on the horizon, I did not have enough time to set up the camera on a tripod, so I steadied myself on the Jeep’s spare tire for a quick, unguided exposure.

© James R. Johnson, 2015

Recommending a Telescope

Q: I have a Canon 5D. Any recommendations for a compatible telescope?

A: You have a high-end camera that deserves high-end equipment. I am so happy with my first telescope/mount combination (TeleVue NP101is/Losmandy GM8) that I continue to recommend it four years after I purchased it. Ultimately, selecting a telescope is a highly personal proposition, and your budget and anticipated uses will determine which is THE right telescope for you. There is a lot of worthwhile material on the Web about the best telescope for particular application.

Here are links to some posts that provide a sense of how I have thought through some of my selection decisions:

Jim’s Astrophotography Equipment
Getting over Aperture Fever
Trying Out a Used Celestron C11
Rationalizing a GOTO Telescope Mount
Finding Barnard’s Star
Planning to Build an Observatory

© James R. Johnson, 2015.