Stars

Sky Charts and Stargazing (for Beginner)

		north or south of this) - suitable for viewing in the United States, Canada, Europe, and Japan. The Southern Hemisphere charts usually depict the sky from a latitude of 35oS. These are for use in the South Pacific, Australia, New Zealand, South America, and southern Africa. The sky charts in Figure 08-01j,k divide the sky of the Northern Hemisphere in January into two quadrants one facing North, the other
Figure 08-01j Sky Chart, North [view large image]	Figure 08-01k Sky Chart, South [view large image]	South. It has a legend to show the various objects in the sky and the apparent magnitude of the objects.

		Figure 08-01l shows the Northern sky at 50o latitude at midnight for the beginning of the four seasons. An one piece sky chart plots the sky with the North Pole at its center (see Figure 08-01m). An oval opening in an overlapping disc represents the heavens as seen from a certain latitude, e.g., 45oN. The time and date of viewing can be selected by rotating the disc around the center. This particular view is set at 22:00 h, January 20. The transparent cursor scale (from -50o to 90o) is used to calculate the declination of celestial objects. The right ascension is marked at the outer-most circle. The East and West are switched in the chart. It will show the correct direction by rotating 180o when it is held over head to compare with the actual sky view.
Figure 08-01l Sky Chart [view large image]	Figure 08-01m Star Finder [view large image]

		Sky charts computer software is perhaps the most versatile. It allows the user to specify any location and date/time as shown in Figure 08-01n, which displays a chart tailored to a "Sample" with latitude 42o and longitude 270o at 22:00 h on January 20, 2004. The detail of objects can be adjusted by the user. It can display the ecliptic as well as the Galactic equator. The coordinate grids can be numbered. Outline of the Milky Way can be plotted on the chart. The name of each object (if not already shown) can be obtained by
Figure 08-01n Sky Chart, Computer Generated [view large image]	Figure 08-01o Sky Chart, Horizon Coordinate [view large image]	clicking the pointer (such as NGC2539 in the sample chart). Figure 08-01o shows the same chart in horizon coordinate facing North. This free sky charts software is offered by Cartes du Ciel.

Stargazing (for Beginner)

Figure 08-01p shows the signposts of 4 different seasons on a particular day, time, location, and direction as indicated in the images. However, the pattern should be approximately valid during the viewing season when adjustments are taken on the shift in day and difference in location. The connecting lines between the brighter stars trace out some kind of object. They are purely artificial pictures making up by observers. The distribution of stars is really random, it requires a lot of imagination to figure out something out of them. The setting of these pictures are translated to Greenwich Observatory (in London) at about 50o Latitude North on the particular day and time (as shown). Figure 08-01q below shows the conversion of Greenwich time (GMT) to various world times. Day time is indicated by white clock, while black clock is for night time.

Figure 08-01p Signposts of the 4 Seasons (Northern Hemisphere) [view large image]

For example, in New York using Eastern Standard Time (EST) which is 5 hours behind GMT, i.e., EST = GMT - 5; thus New Yorker has to wait 5 hours to see the same signposts.



Figure 08-01q GMT Conversion

• The configurations in Figure 08-01p should be valid approximately from Latitude N50^o 15^o. The range of night-time observable latitudes (at Greenwich) should be from the North Pole to S40^o depending on the season.



The viewing direction is indicated at bottom line of each picture. The view is facing that direction with the opposite direction behind (pointing out from the screen and unseen). The daily star trail (via Earth's rotation - counter-clockwise spin) is indicated by the curly arrow (see a star trails video). The visible portion of constellations at night changes with season as the Earth revolves around the Sun (Figure 08-01r). Figure 08-01s shows all the constellations from N50 to S50.

Figure 08-01r Ecliptic [view large image]



Figure 08-01s Constellation Chart (in Mercator Projection which makes Greenland bigger than South America)

Table 01 is a specific example for the yearly cycle at Greenwich (~ London).



Date	Night Time (GMT)	Duration of Night (hours)	Range of Constellations (see Ecliptic)	Range of Right Ascension (RA) (in hour, see Constellation Chart)
March 21	6 pm - 6 am	12	Pisces - Virgo	1 - 13
June 21	8 pm - 4 am	8	Gemini - Sgr. (Leo - Sgr.)	7 - 19 (11 - 19)
September 21	6 pm - 6 am	12	Virgo - Pisces	13 - 1
December 21	4 pm - 8 am	16	Sgr. - Gemini (Libra - Gemini)	19 - 7 (15 -7)

Table 01 Night Sky Over Greenwich Observatory (~ London) During The 4 Seasons

Here's a legend for Table 01:
• Date - The dates are selected close to Spring Equinox, Summer Solstice, Autumn Equinox, and Winter Solstice.
• Night Time (x) - It is in 24-hr format. Use (x - 12) to translate to pm for x > 12 hr (pm switches to am at mid-night).
• Duration of Night - The shorter night time in Summer is fully compensated in Winter.
• Range of Constellations - The shorter and longer ranges in Summer and Winter respectively are shown inside the brackets.
• Range of Right Ascension (RA) - The corresponding shorter and longer ranges of RA are shown inside the brackets.


• Planisphere is a versatile tool to identify celestial objects in night sky. Here is just one example using the night view of the Spring Equinox at 22 hr from Greenwich (see Figure 08-01p).
  • Figure 08-01t,a is a standard view looking North by placing the planisphere overhead with North pointing away, while the Southern horizon is behind with only the Northern half of the night sky in view. The sky is turning counter-clockwise.
  • Figure 08-01t,b is the same sky view but looking South (without placing the planisphere overhead). Now the North points away, it is below the yellow line and unseen. The sky is turning clockwise. This particular view corresponds to the "Spring Signpost" in Figure 08-01p.

  As shown in Figure 08-01t,a, a line drawn from 2 stars in Big Dipper pointing to the North star (Polaris) can serve as the hour hand of a clock. At this particular instance, it is 23 hr in reference to another imaginary line straight up to the North Star; this is called Dipper Time running counter-clockwise in 24 hr divisions. The actual time : t = Dipper Time - 2 x (number of months since March 6). Therefore, on March 21, when the Dipper Time is 23 hr, t = 23 - 2 x (1/2) = 22 hr.

  Figure 08-01t Planisphere View [view large image]

• Star Trails - The stars and all astronomical objects are constantly moving across the sky as the Earth is turning around. The trails

  		depend on the view of the observer as shown in Figure 08-01t2 in which the sense of rotation switches to the opposite according to whether the viewing is from within (the bubble, i.e, the celestial sphere) or from outside (as depicted in the drawing). The video link via Figure 08-01t3 shows the star trails in 4 directions, i.e., West, East, North, and South. Actually, the one for the West is North-West, and the East should be North-East (see the corresponding images). The video was taken in December, at latitude ~ N35.
  Figure 08-01t2 Stars Trails, Northern and Southern Hemisphere	Figure 08-01t3 Stars Trails (viewed from inside) [image, video]

Here's some comments about the four signposts :


1. Spring -
   • Sirius is the brightest star (with apparent visual magnitude m_v = -1.46) in the sky. It had been recognized by ancient civilizations some 3000 years ago. It is about 15^o south of the celestial equator (see Figure 08-01s). Thus, it appears only briefly for 4 months from January to end of April for observer at latitude N50. However, it can be observed further South from late Winter to late Summer.
   • With a little more imagination, the observers can see the parade of the Spring-Summer constellations one after another in the series of Gemini - Cancer - Leo - Virgo along the ecliptic.
   • Arcturus is another bright star (with m_v = -0.05, i.e., about 3.7 times dimmer than Sirius) further East close to Ursa Major. The traditional name "Arcturus" is derived from Ancient Greek meaning Guardian of the Bear, i.e., Ursa in Latin. BTW, the definition of brighter for more negative m_v follows the same logic of more expensive for 1st class, less for 2nd class, ...
   • The relationship between magnitude and brightness (intensity) I_n is expressed by the formula :
     I₂ = I₁ 10^{0.4(m₁-m₂)} .
     For example, if m₁ = +4, m₂ = -1, then I₂ = 100 I₁, i.e., I₂ is 100 times brighter than I₁.
   • On this particular day of March 21, Sirius will disappear to the Western horizon at midnight.
   • Terminator or twilight zone is a moving line that divides the daylit side and the dark night side of the sky. It is not a very shape division as shown in those photos below taken on or near the Spring Equinox.

     		Figure 08-01u shows the terminator at dawn facing South on March 21 with the Autumn-Winter constellations raising up from East to West. They will soon become invisible as the Sun goes up further. Meanwhile, Figure 08-01v captures the Spring-Summer constellations disappearing into the Western horizon at dusk on March 24.
     Figure 08-01u Terminator, Dawn [view large image]	Figure 08-01v Terminator, Dusk	Meanwhile the Milky Way sinks toward the horizon and almost disappears in May (see a "Sky Chart of May" and a midnight photo.

     See "Milky Way Gazing" for detail.


2. Summer -
   • The Summer Triangle has the 3 vertices at Altair (m_v = 0.76), Deneb (m_v = 1.25), and Vega (m_v = 0.026), i.e., 7.7, 12.5, 3.9 times dimmer than Sirius respective (see Figure 08-01p).

   Since it is located at higher Latitude ~ N40 (see Figure 08-01s), Cygnus (the Swan, Figure 08-01w) is one of the most recognizable constellations of the northern summer and autumn. It is visible during the evening hours starting in May (close to the Northern horizon) and continuing on through December. The very best time to see it is during the mid-summer months, when it will be easy to find almost directly overhead on any clear summer night. The body of the swan is inside the "Summer Triangle" with Deneb at its tail (Figure 08-01p). It is also recognized as Northern Cross depending on imagination.

   Figure 08-01w Cygnus

   On this particular day of June 21, Cygnus will remain visible all night.

3. Autumn -
   • This signpost portrays the Great Square (Pegasus) raising up from the Eastern horizon in early Autumn and gradually pushing the Summer constellations off stage.

   Next to Pegasus is the Andromeda constellation with the Andromeda galaxy (M31) barely visible by naked-eyes (mv = +3.44, ~ +6 is the limit). The nature of this astronomical object was finally resolved on 1925 as a galaxy similar to the Milky Way at a distance of 2.5 million light-years from Earth. The Cassiopeia constellation shares the same part of sky with the Milky Way, which is too faint to be seen in Figure 08-01p without long exposure time in photographing.

   Figure 08-01x Cassiopeia

   The Milky Way actually is situated at the left of the picture frame in Figure 08-01p. It bisects the sky running almost due North-South at this time of the year (see Figure 08-01x).



4. Winter -
   • The Orion constellation is the dominant feature in Winter sky. It is visible from November to February in Northern Hemisphere. It is readily recognizable because of the 3 stars lining up in the "belt", which stand out as something regular among the random distribution of stars.
   • The brightest star Sirius lays further south as shown in Figure 08-01s. It has raised up in Figure 08-01p already at mid-night in late December, while it is still below the horizon in early evening in Figure 08-01z below.

   The Pleiades also known as the Seven Sisters, is an open star cluster located further up North in the Northern celestial sphere (see Figure 08-01y circled). It is known since antiquity to cultures all around the world. For a collective magnitude mv = 1.6, at least 5 member stars are visible to the naked eye, while under moderate conditions this number increases to 9, and up to more than a dozen under clear dark skies (see Figure 08-01z facing South). It can be figured out at the Northern end of a straight line connecting with the Orion and Sirius (see the yellow line in Figure 08-01s).

   Figure 08-01y Pleiades [view large image]

   The Milky Way stretches around the Northern half of the Winter sky and disappears from the Southern half as shown in Figure 08-01z below. It can be traced out by looking for Sagtta, Cygnus, Cassiopeia, Perseus, Aurgia, Taurus, and Orion.

   • The North Pole (~ Polaris) is located near the middle of the Winter sky (see the Northern half in Figure 08-01z below) as the chart is produced for latitude N40 (~ half of the range of latitude from 0 to 90 degree).

   

   Figure 08-01z Winter Night Sky (Northern Hemisphere)

Finally, a few words about stargazing in Southern Hemisphere :


• For Southern stargazing, the celestial pole points to the Southern sky so that the stars move in clockwise direction (facing south) while they are still raising from the East and setting to the West (Figure 08-01z2).

		The Southern Polar star is Australe - a very dim star of mv ~ 5.47 near the visible limit. A better way to look for the South pole is to estimate 1/4 distance of a straight line from the Small Magellanic Cloud (SMC) to the Crux (see yellow line in Figure 08-01z3). The arrival dates for the 4 seasons change to September 21 - Spring Equinox, December 21 - Summer Solstice, March 21 - Autumn Equinox, and June 21 - Winter Solstice.
Figure 08-01z2 Stars Trails, Northern and Southern Hemisphere	Figure 08-01z3 Southern Sky, June, Facing South [view large image]

As the Milky Way looks like a round ribbon on the celestial sphere (Figure 08-01v), it has similar appearance to the Southern observers except with 6 months inter-change. For example, it almost disappears in November (instead by May in the North), and in its full glory in June (instead of December; see Figure 08-01z3, and a video via Figure 08-01z4). It is by courtesy of ESO, Atacama at latitude ~ S20. The video records the motion of the Milky Way in June from dusk to dawn. The South pole appears low in the sky near the LMC and SMC with mv = 0.9 and 2.7 respectively.

Figure 08-01z4 Motion of Milky Way [image, video]

The relationship to GMT (Greenwich Mean Time) is unchanged (see Figure 08-01q). The Constellation chart in Figure 08-01s covers equally the Northern and Southern Hemispheres from latitudes N50 to S50.

Milky Way Gazing

In Figure 08-01z51, the Milky Way is displayed on a Celestial Sphere (squashed to show 2 poles and 18 hr). Its movement can be visualized by considering 2 branches hanging from the top (like a cloth hanger); the whole thing rotates in 24 hours cycle. Then it becomes rather obvious to see 2 slant ribbons moving from East to West, and a round ribbon overhead in near circular motion (in December, see the moving Milky Way by clicking Figure 08-01z4) as the globe turns in clockwise direction. The lower South-West part of the Milky Way can be traced out by linking some interesting astronomical objects from the Carb Nebula (mv = 8.4, not visible by naked-eyes) - Betelgeuse (a bright star in the Orion constellation with mv ~ +1) - Sirius (the brightest star with mv = -1.46) - Vela X (a pulsar with mv = +6.9 not visible by naked-eye) - Alpha Centauri (the closest star system to Earth with mv = +0.01). See

Figure 08-01z51 MW on Celestial Sphere

Winter Night Sky for tracing out part of the Northern Milky Way by some constellations. The Milky Way is visible by naked-eyes only in country side away from city light as its mv ~ +5 is near the visual limit. BTW, the original "Sky Chart" in Figure 08-01z51 has something to do with the "Pyramids at Giza".

The 4 illustrations in the followings are very important in understanding Milky Way gazing :


1. Figure 08-01z5 is the view from a planisphere similar to the one in Figure 08-01t but it is set specially on mid-night, June 23 to show some common features of the Milky Way (dotted ribbon) :

   The segment from Capella in the Auriga (Aur) constellation to Deneb in Cygnus is almost parallel to the latitude N45 (for a total of ~ 150o). Thus, it always turns in circular motion. The Sgr and Sco constellations in the Southern Hemisphere are both located ~ S35. They appear on the Southern horizon at the observational limit for N50 observer. The Milky Way in between the above 2 cases runs in an acute angle to the latitudes and drifts across more or less like a slant ribbon over the sky. This is the most frequent viewing pattern.

   Figure 08-01z5 Milky Way à la Planisphere

   The appearance of the Milky May also depends on the observer's latitude. For example at N50, the segment of Milky Way near the pole to declination 50o would turn around in circle, while the rest would drift to the West.

   
   
2. Figure 08-01z6,a is a sideview of the Solar system within the Milky Way by an external observer, e.g., from a spaceship far far away. It is the same placement (as shown in Figure 08-01z6,b and z7) with different perspective. The Solar system is just a tiny spark within a vast expanse of the Milky Way.


3. Figure 08-01z6,b is a Celestial Sphere with the Earth at its center. It is squashed to show both the North and South pole (in full view) and the front face is spread to show an 18 hrs of the sky while back face is magnified to cover the rest 6 hrs. The sense of the Earth's rotation is clockwise so that astronomical objects rise from the East and set at the West.


4. The constellation chart in Figure 08-01z7 is the spherical version of the flat universe in Figure 08-01s. It shows the placement of various objects in abbreviation (see full name in the other chart). The Milky Way is displayed in more detail and accuracy. The Earth at the center also spins in clockwise direction.

Figure 08-01z6 Milky Way

Figure 08-01z7 Sky Chart

Date	Night Time (GMT)	Milky Way Segment	Comments
March 21	6 pm - 6 am 12 hrs	At dusk,    Sirius (SE horizon, in CMa) - Cas (W Zenith) - Cyg (N horizon). At dawn,    Aur (N horizon) - Cas (E Zenith) - Sco (SW horizon). See illustration in September 21.	In between this initial and final configurations, the segment from Mon to Cas swings to the West while the part from Aur to Cas turns in near circle around the North pole carrying Sco at its tail end to the West. See Figure 08-01z6,b to visulize. See a video of Milky Way - March
June 21	8 pm - 4 am 8 hrs	At dusk,     Aur and Cas (N horizon) - Oph (SE). At dawn,     Aur and Cas (N Zenith) - Oph (SW). See illustration in December 21.	In between this initial and final configurations, the segment from Aur to Cas turns in near circle around the Northern pole while carrying Oph to SW. See a "Milky Way - Summer" video facing South.
September 21	6 pm - 6 am 12 hrs	At dusk,     Sco (SW horizon) - Cas (E Zenith) - Aur (N horizon). At dawn,     Cyg (N horizon) - Cas (W Zenith) - Sirius (SE horizon, in CMa). [view large image] Milky Way Views of March 21 and September 21.	In between this initial and final configurations, the segment from Sco to Cas winds up from SW while the part from Cas to Aur turns in near circle around the North pole bringing Mon back to SE. See Figure 08-01z6,b to visulize. Note that the dusk and dawn patterns are the reverse of those for March 21 (same for December/June, 21). See Figure 08-01x for a view of the Milky Way perpendicular to the horizon.
December 21	4 pm - 8 am 16 hrs	At dusk,     Oph (SW) - Cas and Aur (N Zenith). At dawn,    Oph (SE) - Cas and Aur (N horizon). [view large image] Milky Way Views of June 21 and December 21.	In between this initial and final configurations, the segment from Oph to Aur turns around the North back to SE, while Cas and Aur moves in an arc around the Northern pole. This is the rare occasion when the Milky Way is easily visible as an arc over the sky. See a "Milky Way - Winter" video facing North and a grand finale in 4 directions. BTW, the first painting featuring a prominent Milky Way is probably the "Flight into Egypt" of the holy family by Adam Elsheimer in 1609. It clearly depicts a Winter scene (~ Christmas time) as the Milky Way is in the form of an arc over the sky.

Table 02 Milky Way Gazing

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