What is the difference between latitude and declination

Had you moved south, the opposite would have occurred as the Celestial Equator moved north of the zenith. As we move north and watch the sky, we notice that some stars circle the NCP endlessly, never getting below the horizon, and that some stars vanish from our sight, never getting a chance to rise.

The stars of the first group are called circumpolar stars. Since the NCP is 30 degrees from the horizon, it makes sense that stars close to the pole -- within 30 degrees -- will never drop below the horizon as they circle the NCP. Similarly, stars with declinations ranging from degrees SCP to degrees never rise above Austin's horizon. The southernmost stars that can be seen from Austin have declinations of about degrees although hills, buildings and haze make degrees a more practical limit.

So we can say in general:. As another example, consider an observer in Tasmania, at 40 degrees S latitude. This observer would see the celestial equator 40 degrees about 4 outstretched fists, remember to the north of the zenith. No star marks the SCP, but the stars would seem to circle a point 40 degrees above the point of due south. This observer would never see the Big or Little Dippers, but would see the constellation Scorpius pass almost directly overhead!

Just as we measure declination in degrees north or south of the Celestial Equator, so do we measure RA east of a point called the Vernal Equinox. The Vernal Equinox is the location in the sky where the Sun can be found on the first day of Spring the day is called the Vernal Equinox as well. The Sun moves through the constellations of the Zodiac over the course of the year, but the location of the Vernal Equinox stays roughly constant, and the Sun returns there every March We typically do not measure RA in degrees, but rather in hours.

The reason we do this is to assist us in timing our observations. While we use a physical location on Earth as our reference for longitude, what reference do we use for right ascension? Astronomers use the spot the Sun arrives at on the first day of spring, called the vernal equinox. Presently, it's located in the constellation of Pisces, the Fish.

The sky can be treated as a clock, since it wheels by as Earth rotates, so the zero point of right ascension is called "0 h " for "zero hours. That's a little more than one-half the width of the W -shaped constellation Cassiopeia. In keeping with right ascension's time theme, hours are subdivided into minutes and seconds, and are even written out as minutes m and seconds s. Let take the North Star for example.

Polaris is located at RA 2 h 41 m 39 s , Dec. Because the stars circle about the sky every 24 hours, right ascension or RA ranges from 0 h to 24 h. Unlike Earth coordinates, celestial coordinates change due to the slow wobble of Earth's axis called precession. Precession causes the equinox points to drift westward at a rate of As the equinox shifts, it drags the coordinate grid with it.

That's why star catalogs and software programs have to be updated regularly to the latest "epoch. Most catalogs and software currently use Epoch J The next major update will happen in Learning RA and Dec. Before computer software effortlessly plotted the paths of newly discovered comets and fast-moving asteroids, I couldn't wait to get my hands on their coordinates. I'd hand-plot the positions on a paper star atlas, then swing my scope to the spot, and thrill when I found it on my own.

RA and Dec. Just input its coordinates, hit enter, and you're there. If you hear of a new comet or fast-moving asteroid, a quick check of its changing coordinates will tell you not only where it is but where it's headed, so you can plan the best time to see it. I made friends with right ascension and declination long ago.

Knowing I could drive anywhere on its invisible roadways helped me, and it'll help you too become more familiar with the night sky. Good review and I enjoy RA and Dec. Much of my observing is with a Telrad. You can convert RA and Dec. Log in to Reply. Bob King said: I'd hand-plot the positions on a paper star atlas, then swing my scope to the spot, and thrill when I found it on my own.

I don't know how to take this step with my Dobsonian. Do I need some add on protractor etc? How do I find the vernal eqinox? Not much sky watching this winter. Stew Shouldice. Bob King Post Author. Hi Stew, I also use a Dob and have for many years.

You don't need to know where the vernal equinox is if you're hand-plotting. You get the specific RA and Dec. If it's a bright star, then that's easy, you look to see if that star is up in the sky at the time you want to view it.

You can do that by using a planisphere star-wheel or a free software planetarium program like Stellarium. If you're looking for a fainter object, you'll need a star atlas.

Star atlases will always shows right ascension along the bottom and declination along the side. You locate a specific object by interpolating between those numbers. Larger atlases come with their own separate fine-gradation grids you can place over the page to really nail the position down. Does this help?

Can someone show me the formula to resolve altitude and azimuth of polaris using observer latitude, longitude, elevation above sea level, UT?

It is interesting that Jean Meeus Astronomical Algorithms does not even have an index entry for polaris. Bob, this explanation would fit nicely in the first chapter of your next book! Lately I've been figuring out how to use the northern circumpolar stars and the Sun's approximate right ascension to tell time at night. Put Polaris at the middle of your imaginary clock face, find whichever of these stars are visible, and interpolate the rest of the clock.

The Sun is at 0 hours RA at the March equinox, 6 hours at the June solstice, 12 hours at the September equinox, and 18 hours at the December solstice.

Figure out how far we are between an equinox and a solstice and interpolate. Astrological signs are easier, because every astrological sign is about two hours of right ascension wide, starting with 0 h at the first point of Aries, 2 h at the start of Taurus, etc. The next step is to deduce the angle between the Sun and a convenient circumpolar star, and finally to figure out where the Sun's right ascension is on your circumpolar clock.

Imagine the Sun moving below the horizon from your western horizon toward the eastern horizon, and that allows you to "see" where the Sun is at any given moment from sunset to midnight and from midnight to sunrise. The Sun is currently about one week into the sign of Pisces, or three weeks before the March equinox, so the Sun's right ascension would be about 22 hours 30 minutes according to theskylive. In other words, the Sun is about 30 minutes west of being opposite Dubhe and Merak.

So if it's nighttime and Dubhe and Merak are east of the meridian, the Sun is west of the meridian, i. When Dubhe and Merak are transiting the meridian it's 30 minutes after midnight, and when Dubhe and Merak are west of the meridian the Sun is east of the meridian, i.

By estimating the angle between the pointer stars and the meridian and adding the equation of time , I find I can estimate the time to within 15 or 30 minutes. The current equation of time is minutes 45 seconds , so subtracting a negative number means that clock time is about 13 minutes later than local solar time.

You also need to correct for your longitude east or west of your standard time zone's central meridian. For each degree east of your central meridian your clock time is four minutes earlier than mean solar time; for each degree west of your central meridian your clock time is four minutes later than mean solar time.

Here in San Francisco we are 2 degrees 26 arcminutes west of the central meridian, which means our clock time is an additional ten minutes later than mean solar time. Thanks, Anthony! Please sir can you explain to me how I can convert RA in degree minute second to arcsecond. You can convert R. But, when I think about this a bit deeper, I start to miss something. Seasons result from the The seasons are obviously opposite in the Earth's two hemispheres.

The polar direction is slowly changing due to precession direction takes 26, yrs to complete one circuit but the angle between the polar axis and the ecliptic is fixed and invariant. We also tend to identify various constellations with the seasons because the stars visible at night change throughout the year. The ecliptic passes through the constellations of the zodiac. The phases of the moon repeat with a month time scale as the Moon orbits the Earth. Positional Astronomy. Astrometry: the science of measuring stellar positions very accurately.

When making precise measurements of stellar positions, various effects must be taken into account:. By making accurate position measurements, we have discovered a number of interesting effects such as precession which is caused by the torque of the moon on the Earth. The direction that the Earth's polar axis is pointing in space is slowly changing and describes a circle on the sky and it takes the polar direction 26, years to travel this circle once.

The location of the Vernal Equinox shifts by 50" per year. Precession also means that positions of astronomical objects are tabulated for a stated time such as Eclipses occur when the Earth, Sun, and Moon lie along a line such that either the shadow of the Earth falls on the Moon lunar eclipse or the Moon's shadow falls on the Earth solar eclipse.

Note that lunar eclipses can only occur at full moon and conversely, solar eclipses can occur only at new moon. Why don't eclipses occur every month? The line connecting the two intersection points between the Moon's orbit and the ecliptic plane is called the line of nodes.