česky english guestbook guestbook

Getting oriented in the night sky

Today's interactive computer sky maps (planetarium software, app on a smart phone or something like a sky scout) are an almost indispensable adjunct to traditional printed charts. But even if you prefer computer use, map reading is still an important skill to have, and it's easy to learn. It makes our recreational astronomy a bit deeper and stargazing more challenging. To understand the context of how to get oriented in the night sky, focus on the following topics. Don't be intimidated by them, it really gets easy with practice!

How did it all begin?

Motion of the Sun in the sky and the brightest stars in the night sky and motion and phases of the Moon are repeating cycles regularly observed by ancient cultures. Man's journey toward the relatively precise calendars and mutually bounded early beginnings of astronomy is based upon these simple observation with the naked eye – which is one of the most fascinating aspects of this period and, at the same time, one of the greatest adventures that the human mind has ever begun!

The Earth turns once in about 24 hours with respect to the Sun, that almost everybody knows. One more interesting fact about Earth rotation is less known: the Earth needs only roughly 23 hours 56 minutes to rotate by 360 degrees! This time, however, it is taken with respect to the distant stars. Because of the Earth's revolution around the Sun, a solar day is nearly 4 minutes longer than a sidereal day! Why is this important for us to understand? As a result, the stars appear to rise, culminate, and set 4 minutes earlier each subsequent night! And what happens after a month? Any given star will rise and then later cross your meridian 2 hours earlier than it did the month before. At last, after a year, the stars will occupy the same relative positions in the night sky…

Now we also know why some constellations are visible only during certain seasons. If we understand this principle we have a master key that opens many doors in the recreational astronomy!

Nowadays, although being not an essential part of life, telling time by the stars at night is still very educational. It teaches us to understand the apparent motion of the night sky in reality. The following short manual requires a bit of imagination, simple arithmetic and practice and is limited to northern latitudes in which Cassiopeia never dips below our horizon.

For those interested, here are some useful basic terms and definitions:

  • Zenith is the highest point of the celestial sphere directly above your head.
  • Meridian is the circle passing through your zenith and the horizon's north and south points.
  • Solar Time is based on the apparent motion of the Sun across the sky.
  • Sidereal Time is based on the apparent motion of the distant stars across the sky.
  • Solar Day is based on the length of time it takes for the Earth to spin around once on its axis with respect to the Sun.
  • One Solar Day = 24 hours
  • Sidereal Day is based on the length of time it takes for the Earth to spin around once on its axis with respect to the distant stars so that a distant star appears in the same position in the sky.
  • One Sidereal Day = 23 hours 56 minutes 4 seconds
  • Vernal Equinox in the Northern Hemisphere occurs around March 20 or 21 when the Sun crosses the celestial equator south to north. During this period of time, the time difference between sidereal time and solar time is close to 12 hours and equal to this value at one particular time.
  • Autumnal Equinox in the Northern Hemisphere occurs around September 22 or 23 when the Sun crosses the celestial equator north to south. During this period of time, the time difference between sidereal time and solar time is close to 0 hours and equal to this value at one particular time.
  • A light-year, used informally to express astromical distances, is the distance light travels in one year: 1 ly ≈ 10,000,000,000,000 kilometres

 

A Solar Analemma is a diagram that depicts the Sun's position in the sky at the same time of the day during a year at a specific location. The graph was completed using Stellarium. The photo was taken from a viewpoint near Vlachovice, Nové Město na Moravě, at 7:12 GMT, lens 8mm.   

To specify positions of celestial objects, there is a complete analogy between the geographic and celestial coordinate systems. Among other reference points, the celestial sphere has a north and south celestial pole and a celestial equator which are projected reference points to the corresponding positions on the Earth. Right Ascension (RA) and Declination (Dec) serve as an absolute coordinate system on the sky as Longitude and Latitude do on the Earth surface. Right Ascension is related to the rotation of the Earth, so it runs from 0 to 24h. Declination is the equivalent of Latitude measured in degrees from the celestial equator (0 to -90° and 0 to +90°)

A useful result of measuring the right ascension in time is that a celestial body culminates when the local sidereal time is equal to its right ascension.

When is the best time to observe the Lagoon nebula in the constellation Sagittarius?
The best time for stargazing is around and after midnight when the sky is the darkest. At the time of the Autumnal Equinox, the sidereal time is nearly 0 hours at local midnight. The right ascension of the Lagoon nebula is 18h. The every month the stars and the Lagoon nebula would be further to the westward in the night sky by about 30 degrees, representing 2 hours. 18 divided by 2 is equal to 9; 9 months after the autumnal equinox. It will be well‑placed in Jule and August!

Some quintessential deep sky objects are both a blessing and a curse for observes at our northern latitudes: Being at a low declination, below celestial equator, they hardly ever really emerge from the light pollution “soup” along the horizon!

We are going to estimate the local sidereal time and simply convert it to solar time. Solar time is the basis for civil time and standard time, the same as kept by your wristwatch.

Why is the Cassiopeia constellation appropriate for telling the time by the stars at night? Beta Cassiopeiae, Caph or Al Sanam al Nakah is a good rough indicator of sidereal time, because a line drawn through Polaris and Caph passes close to the vernal equinox point; its right ascension is 0h 09m. Hence, when Caph is on the local meridian the local sidereal time is roughly 0 hours (more precisely 0:09 hours).
  • Find Polaris, the North Star.
  • Find Cassiopeia. The brightness of its stars and the clear “w” shape makes Cassiopeia an easy recognizable constellation.
  • Consider the North Star as the center of a clock and draw an imaginary hour hand from the North Star to Beta Cassiopeiae (Caph).
  • Now imagine a 24-hour clock face, where 0 or 24 is towards the zenith, 6 is towards west, 12 is opposite 0 or 24 and 18 is towards east. The hour hand moves counterclockwise!
  • When the hour hand of our sidereal clock points straight to 0 or 24 at the time of the Autumnal Equinox, the solar time is also nearly 0 hours.
Solar Time [hour] ≈ Our_Sidereal_Clock_Time [hour] - 2x Number of months since the AE
On May 7, our sidereal clock will say that it is 12 hours. What is the solar time?
Number of months since AE on September 22: 7.5 months
12 - 2 x 7.5 = -3, 24 - 3 = 21, thus the solar time is 21 hours. The Daylight Saving Time is roughly 22 hours.

As stated before, the main purpose of this example is to elucidate the apparent motion of the night sky. The principle of this calculation is also used to make a planisphere working. Being the best type of star map to start out with, a planisphere or star wheel is a real analog computer for calculating the positions of the stars, designed for specific latitude zones.

I hope the acquired knowledge will be a huge help with your stargazing sessions. Because understanding the positions of the stars, constellations and Deep Sky Objects you see in your binoculars is sometimes as much fun as seeing them! Why should we reveal the secrets of the Deep Sky object for us and thus to enrich our experience of the night sky? What are they & Why observe them? Deep Sky Objects are astronomical objects other than Solar system bodies and individual stars. The classification is used for the most part by amateur and recreational astronomers to denote visually observed faint naked eye and telescopic objects such as star clusters, nebulae and galaxies.

  • A star cluster is a group of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds or thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain fewer than a few hundred members, and are often very young.
  • A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases.
  • A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.

Some of the very simplified definitions above are excerpted from textbooks but are also based on my interpretation of the information available. I'm too lazy to invent a brand new but very similar descriptions.

But the most important question is the last one: Why observe them? Using few words and thus being laconic, I suspect the answer is different for each of us and it is also constantly changing for each of us.

Theoretical physicist Lawrence Krauss puts it in a poetic way: “The amazing thing is that every atom in your body came from a star that exploded. And, the atoms in your left hand probably came from a different star than your right hand. It really is the most poetic thing I know about physics: You are all stardust...” And, as hard as it might be to believe, some of the hydrogen atoms in your body are primordial ones originated billions of years ago in the explosive aftermath of the Big Bang!

Thus, looking into the universe is definitely an uplifting experience!

Wandering through the universe with binoculars

In fact, even small binoculars can reveal many sights that most folks think require a large telescope. Binoculars give a wide right-side up image, making celestial objects easier to find. Let you use both eyes, providing surer, more natural views. Your brain is wired to process two visual channels, enhancing resolution and contrast.

There are a lot of discussions going around on how to select a pair of portable binoculars.

For astronomical work your binoculars should have objective lenses with a diameter of at least 40 mm and a magnification of 8; engraved on the back is “8x40”. Based on my experience over the years, I consider a compact 10x42 or a hair bigger 10x50 binoculars to be of a well chosen size. If the magnification factor is higher than 10, you won't be able to hold them steady enough to get a stable image without using a tripod.

Start your visual odyssey by observing the Moon. Oddly enough, the best time to observe the Moon is a few days after the First Quarter Moon, but before the Full Moon. Try to understand more about the moon's phases. Observing the region along the division between the illuminated and dark parts of the Moon will reveal a host of fascinating features!

Watch the Galilean moons of Jupiter! Try to observe their orbits as Galileo did and sketch the pattern made by the satellites.

Look at the night sky with whatever binoculars you have, the only trick is to avoid cheap plastic toys here!

*

So far so good. All this made us bold enough to take a big step toward capturing the wonder and delight of exploring the farthest reaches of the deep sky near new moon.

Although the brightest deep sky objects can also be spotted with the naked eye, even a small binoculars can take you much deeper into the world of outer space.

The first view you get isn't always the most you can see. The more you observe, the more you see.

However, it is very important not to expect to see deep sky objects as they are displayed in most photographs acquired with very long exposures! Amateur astrophotography also produces some detailed photos that show much more than can be seen with the naked eye!

Here are a few basics to remember as you proceed:

  • Plan your deep sky explorations close to a new moon to prevent a bright moon from interfering with your stargazing.
  • Find clear sky.
  • Choose higher locations if possible; fog tends to form in low lying areas at first.
  • Wait until the sky is dark enough after sunset.
  • Stop hunting when the Moon rises in the eastern sky.
  • Turn off your bright lights, allow your eyes to dark adapt for several minutes at least.
  • Observe deep sky objects when they are on or near the meridian, there will be less atmosphere, dust and humidity light travels through to reach the eyepiece.
  • Cup your hands around the eyepiece to block peripheral light, even from dark-sky areas.
  • Employ an observing technique called averted vision, the art of looking slightly to the side of a faint object being observed!
  • Move the binoculars, very faint details can become evident when the object is being viewed with averted vision, and also moving!

Now, walking somewhere under the night sky to find a place of rest, stop and realize how perfect a moment is: Your binoculars, in enabling you to look far beyond our Milky Way Galaxy, also allows you to look back in time. Light travels at about 300,000 kilometres per second. When you look up into the night sky, you are not seeing the Andromeda Galaxy as it currently is but as it was about 2,500,000 years ago, since it takes that long for the light radiating from this galaxy to travel nearly 24,000,000,000,000,000,000 kilo­metres to Earth.

*

Staying in a fixed position relative to the stars, the Deep Sky Objects are often faint, fuzzy patches or balls of light in any binoculars. Now what can you hope to see? Probably less than you were expecting, but the disappointing first impression will quickly come to past once you start realizing what you are looking at!

When you first start trying to find your way around the night sky, it may seem a bit daunting and discouraging. Without the aid of computers, the way to avoid such disillusionment is to learn a simple and intuitive method called star-hopping. Star-hopping is the technique of moving your binoculars' field of view from one certain known star pattern to another known star field until you get close enough to your target object. Finally, try moving the binoculars slightly up and down and left to right to find what you want. Good 10x magnifying binoculars offer a large field of view that is about at least 6 degrees of arc!

  • Find the nearest bright guide star pattern. The “teapot” in the constellation Sagittarius is one of the most illustrative examples.
  • Find ways to sweep your binoculars from that last star field to the target object.
  • Even a little preparatory homework beforehand using a night sky map will go a long way to making your observing sessions productive and fun!

The star-hopping maps in the appendix are intended to be used as a supplement to a planisphere and to the more thorough and detailed sky charts.

Now let us go ahead to the distant night sky landscapes... There are countless of them. We mainly focus on the brightest Deep Sky Objects which can occasionally be glimpsed by the naked eye, but it's always sort of amazing what can be seen even with small binoculars! All but one are included in the Messier catalogue, thus the letter M is prepended to the number. NGC is the abbreviation for New General Catalogue.

M31: The Andromeda galaxy, our Milky Way's nearest large neighbor, is one of the farthest objects in the clear and dark night sky that you can see with naked eye. In a pair of 10x42 or 10x50 binoculars it could be easily seen as 2‑degree wide elliptical object with a very bright core. Distance: 2.5 Mly (millions ly);
RA = 0h 43m; Dec = +41.3°

NGC 869 and NGC 884: The Double Cluster, Caldwell 14. Being close together in the constellation Perseus, they form a famous showpiece object that is even easily spotted with the naked eye and thus act as a fantastic binocular target. As an aside, to get an idea of the intrinsic brightness of the stars that you see in this Double Cluster, if our Sun was at the same distance as the Double Cluster, it would be too faint to be seen in10×50 binoculars! Dist. 7,500 ly; 2h 20m; +57.1°

M45: The Pleiades, or Seven Sisters, is an open star cluster containing middle‑aged hot stars; it is the cluster most obvious to the naked eye in the night sky! Putting binoculars onto it is akin to entering a cave of blue diamonds as significantly more of its blue-white stars are revealed. Dist. 440 ly; 3h 47m; +24.1°

M42: The Orion nebula is a diffuse nebula in the constellation of Orion, laying south of its Belt. Once you find the Belt stars, you can also locate the Orion Nebula, otherwise known as M42, a stellar nursery where new stars are being born. In a pair of 10x50 binoculars and by using averted vision, a wealth of detail becomes apparent. The longer you observe, the more you see! Dist 1,340 ly; 5h 35m; -5.4°

M81: A spiral galaxy in the constellation Ursa Major. M81 is also known as the Bode's Galaxy. The galaxy's large size and relatively high brightness make it a perennial target for binoculars. Bode's Galaxy apparent size corresponds to a spatial diameter of 90,000 light years which makes it about three-fourth in size compared to our Milky Way. A supermassive black hole lurking at the center of the Bode's Galaxy has been found to be 70 million solar masses, or 15 times the mass of the Milky Way's black hole. Dist. 12 Mly; 9h 56m; +69.1°

M82: A starburst galaxy in the constellation Ursa Major. M82 is also known as the Cigar Galaxy. Not surprisingly, the starburst activity is thought to have been triggered by interaction with neighboring large galaxy M81. M82 has recently been in the limelight for Dr Fossey's serendipitous discovery of the type Ia supernova SN 2014J. This supernova ejects the bulk of the chemical elements during its explosion including iron – the same iron that provides the protein hemoglobin in your red blood cells with the ability to transport oxygen to the tissues of your body, making carbon-based life possible! Dist. 12 Mly; 9h 56m; +69.7°

M4: An old globular cluster in the constellation of Scorpius, a stellar graveyard. It is easy to find, being located 1.3 degrees west of bright orange Antares. Many of its more massive stars have made the transition to their final white dwarf stage. A white dwarf is the burned-out core of a collapsed star that slowly cools and fades away. Thousands of white dwarfs the cluster is predicted to contain. M4 is conspicuous in a pair of 10x42 binoculars as a fuzzy ball of light. It appears about the same size as the Moon in the sky. Dist. 7,200 ly; 16h 23m; -26.5°

M13: The Great Globular Cluster in the constellation Hercules is very often considered to be the most impressive globular cluster in northern skies. In a pair of 10x42 or 10x50 binoculars, this cluster looks somewhat like a hazy mothball, resembling a comet without a tail in appearance. Dist. 22 kly; 16h 42m; +36.5°

M20: The Trifid Nebula is a remarkable and very complex object in the constellation Sagittarius. Within it is a stellar nursery, a star cluster of the recently born stars; a bright red hydrogen emission nebula, a blue reflection nebula; and the interesting dark nebula dividing the whole target into the three-part structure that gave the nebula its name. Thus the name Trifid means divided into three lobes. Viewed through a pair of 10x42 and 10x50 binoculars, the Trifid Nebula is a very impressive object!
Distance 5,200 ly; 18h 02m; -23.0°

M8: The Lagoon Nebula is a large gas cloud within the Milky Way Galaxy, a stellar nursery; the largest and brightest of a number of nebulosities in and around Sagittarius, barely visible to the unaided eye if it is high enough in a clear and dark sky. The complex nebulosity is visually about three times the size of the full moon and benefits greatly from averted vision! Distance 4,100 ly; 18h 04m; -24.4°

M16: The Eagle nebula in the constellation Serpens. This famous nebula contains several active star‑forming gas and dust regions, including the Pillars of Creation photographed by the Hubble Space Telescope in incredible detail. Their name is very apt as the gas and dust are in the irreversible process of creating new stars, while also being heavily eroded by the light from nearby stars that have recently formed. A pair of 10x50 binoculars will show a very faint and fuzzy triangular shaped patch of light with the brightest stars resolvable. 7,000 ly; 18h 19m; -13.8°

M17: The Omega Nebula in the constellation Sagittarius, also known as the Swan Nebula. With its local geometry being similar to the Orion Nebula (except that it is viewed edge-on rather than face‑on), M17 is considered one of the brightest and most massive star-forming regions of the Milky Way. Under closer examination with averted vision an extension of an elongated greyish fuzzy patch appears, giving the nebulosity the appearance of a tick rather than a swan. Distance 5,500 ly; 18h 20m; -16.2°

M11: The Wild Duck cluster is a very rich and remarkably compact open cluster in the constellation Scutum, containing about 3000 stars. Distance 6,200 ly; 18h 51m; -6.3° 

M27: The Dumbbell nebula. A planetary nebula, more correctly known as a stellar‑remnant nebula is believed to be a normal stage in stellar evolution involving the expulsion of stellar material back into space. M27 lies in the constellation of Vulpecula, just over 3 degrees north of gamma Sagittae. The nebula is easily visible in 10x50 binoculars, appearing as a very small oblong patch of light. Distance 1,300 ly; 20h; +22.7°

We stick to a few simple rules when observing, and, although they appear as faint, hazy elongated smudges of light, we see them! 

<< Go to homepage

Published on  November 30th, 2017

© 2019 - bloumani.cz
get-simple.info