Documents & Resources - Search Results
Search results
Serpens Cauda Constellation Map
image
Caption: A map of part of the constellation Serpens. Serpens is unique amongst IAU constellations in that it is divided into two non-contiguous parts. The part of Serpens shown here is known as Serpens Cauda, representing the tail of the serpent. This part of Serpens is surrounded by (going clockwise from the top): Ophiuchus, Sagittarius, Scutum and Aquila.
Serpens Cauda spans the celestial equator thus parts of the constellation are visible at some point in the year across the Earth. The whole constellation is visible to all but the most arctic and antarctic regions. Serpens Cauda is best viewed in the evening in the northern hemisphere summer and southern hemisphere winter.
The Eagle Nebula and its associated star cluster are marked here as M16 with a yellow circle. This region was the location of the famous “pillars of creation” image taken by the Hubble Space Telescope.
The y-axis of this diagram is in degrees of declination with north as up and the x-axis is in hours of right ascension with east to the left. The sizes of the stars marked here relate to the star's apparent magnitude, a measure of its apparent brightness. The larger dots represent brighter stars. The Greek letters mark the brightest stars in the constellation. These are ranked by brightness with the brightest star being labeled alpha, the second brightest beta, etc., although this ordering is not always followed exactly. The dotted boundary lines mark the IAU's boundaries of the constellations and the solid green lines mark one of the common forms used to represent the figures of the constellations. The blue line at the bottom of the diagram marks the ecliptic. Neither the constellation boundaries, nor the lines joining the stars, nor the ecliptic, appear on the sky.
Credit: Adapted by the IAU Office of Astronomy for Education from the original by the IAU and Sky & Telescope
Credit Link
Glossary Terms:
Apparent Magnitude , Celestial Coordinates , Celestial Equator , Constellation , Declination , Nebula , Right Ascension (RA) , Open Cluster , Sagittarius
Categories:
Naked Eye Astronomy
License: Creative Commons Attribution 4.0 International (CC BY 4.0) Creative Commons Attribution 4.0 International (CC BY 4.0) icons
File
( image
103.44 kB)
PDF File
(PDF file
218.49 kB)
Annual Parallax
image
Created for the OAE
Caption: Distance determination has historically been a challenge for astronomy. One of the primary ways to measure distance is to use annual parallax. The Earth orbits around the Sun over the course of a year meaning that it moves from one side of the Sun (shown here as position A) to the other side of the Sun (position B) over the course of six months. It then moves back to its original position over the remaining six months. This movement subtly changes the perspective an observer on Earth sees the night sky from. This is similar to the change in viewing perspective you may get when viewing a scene from your left eye and then your right eye. The change of viewing perspective causes nearby objects to shift in position in your vision. The annual motion of the Earth around the Sun changes the perspective of the observer enough to shift the observed positions of celestial objects. How big this effect is depends on the distance to the celestial object. Nearby stars will have bigger shifts in observed position than more distant stars.
The positional shift is known as the trigonometric or annual parallax (which we will call α here) and is defined as the shift in position of a star compared to what an observer at the center of the Solar System (the Sun) would see. In this diagram we see the star viewed from perspectives six months apart (positions A and B). When observed from position A the star’s shift in position will be α while when observed at position B it will be –α. Thus the relative difference in the stars position between being observed at position A and position B will be 2α.
The size of the trigonometric or annual parallax in arcseconds is approximately 1 divided by the distance in parsecs. An arcsecond (often represented by a ″ symbol) is the angular diameter a one-metre-long stick would have when viewed from 206 km away. A parsec (often abbreviated to pc) is 3.26 light years or 30.86 trillion kilometres. This is 206,265 astronomical units (the typical distance between the Earth and the Sun). No other star is closer than 1 pc to the Sun so all stars in the sky have trigonometric parallaxes less than one arcsecond.
While trigonometric parallaxes have long been used to measure the distances to objects in our Solar System or nearby stars, recent advances have pushed the boundaries of these distance measures further. The Gaia satellite has pushed the boundaries of parallax measurements to over a thousand parsecs. Arrays of radio telescopes can also very accurately measure the positions of very distant objects and thus their trigonometric parallax.
Note the Earth and Sun are not to scale here and the Earth’s axial tilt is not accurately represented.
Credit: Aneta Margraf/IAU OAE
Glossary Terms:
Angular Diameter , Annual Parallax , Astronomical Unit , Parallax
Categories:
Observational Astronomy
License: Creative Commons Attribution 4.0 International (CC BY 4.0) Creative Commons Attribution 4.0 International (CC BY 4.0) icons
File
( image
1.43 MB)
PDF File
(PDF file
248.89 kB)
Also available in black and white
File
( image
1.43 MB)
PDF File
(
248.89 kB)
Celestial Coordinates
image
Created for the OAE
Caption: Here we see how celestial coordinates map positions on the celestial sphere. The celestial sphere is an imaginary, hollow sphere that surrounds the Earth. Celestial objects appear to be attached to the inside of the celestial sphere, and the planets, Sun, and Moon appear to move slowly across it.
The celestial sphere has a spherical coordinate system similar to the latitude and longitude we see on the Earth. The celestial equator is the projection of the Earth's equator onto the celestial sphere while the north and south celestial poles are the projections of the Earth's north and south poles onto the celestial sphere.
Declination is similar to latitude on the Earth in that it marks the angle above and below the celestial equator. Declination is zero on the celestial equator, is positive to the north of the celestial equator and negative to the south of it. It is commonly measured in degrees and sometimes in radians.
Right Ascension is similar to longitude as it marks the angle around the celestial equator. As with longitude the zero point of right ascension (shown here as a dashed line) is a matter of choice. Traditionally the zero point of right ascension was the position of the Sun at the northern hemisphere vernal equinox (spring equinox) in March. It is positive and increases to the east, unlike longitude on Earth which is defined as east or west of the prime meridian. It is most commonly measured in hours, minutes and seconds or occasionally in degrees or radians.
The ecliptic marks the Sun's path across the celestial sphere when viewed from the Earth.
The Earth rotates within the celestial sphere. This leads to objects like stars or galaxies, which appear static on the celestial sphere, rising and setting when viewed from the Earth but remaining in the same position on the celestial sphere.
The Earth's axis precesses within the celestial sphere. This slowly moves the celestial equator and the celestial poles meaning that the traditional celestial coordinate system changes slowly over time meaning that stars and galaxies that do not move will have positions that change. Astronomers have used coordinate systems defined by the position of the celestial poles and equator at a fixed point in time. Now celestial positions are most commonly measured on a fixed coordinate system defined by the position of the celestial poles and equator on the 1st of January 2000.
Credit: Maria Cristina Fortuna/IAU OAE
Categories:
Naked Eye Astronomy
, Observational Astronomy
License: Creative Commons Attribution 4.0 International (CC BY 4.0) Creative Commons Attribution 4.0 International (CC BY 4.0) icons
File
( image
517.83 kB)
PDF File
(PDF file
203.59 kB)
Also available in black and white
File
( image
501.79 kB)
PDF File
(
202.13 kB)
