Understanding and Using the Celestial Coordinates

We get asked all the time, “how did you know that was there?!” “How did you know that is a planet?” Repeated observations help, as well as getting to know the constellations, but it also helps to understand how things move in the sky.

If you have the basics down, then you are ready for the concept of the Celestial Sphere. In this article, you are going to be introduced to the coordinate system, and learn what they mean. Understanding these concepts help you pinpoint where things are!

Think of the Sky as a Celestial Sphere

the-celestial-sphere-1-638

While we know today that the stars are various distances away, older universe models imagined the sky as one fixed celestial sphere. They were able to figure this out by simple observations of the sky such as:

  • The Sun, Moon, and Stars rising in the east and setting in the west.
  • The stars in the northern hemisphere appearing to move counter-clockwise
  • The stars in the southern hemisphere appearing to move clockwise
  • Certain stars being visible in different positions at different latitudes.
  • The appearance of two celestial poles, one over each hemisphere, and a celestial equator.

So it’s okay to imagine the sky as a celestial sphere above your location.

The Celestial Coordinates

The celestial sphere, when it comes to the sky, has its own coordinate system and methods of measurement! While it’s not necessary to learn this for naked eye observing, they do become important when you get into telescopes, especially equatorial telescopes!

Right Ascension – The Sky’s Longitude

The path that essentially matches Earth’s rotation is called Right Ascension. Although it can sound confusing at first, it’s best explained that RA is measured in units of hours (h), minutes (m), and seconds (s). As an example, Sirius location in RA is 06h 45m 08.917s. From 0h  to 24h RA, it is divided into 24 hourly “wedges” with the 24th hour completing a full circle and resetting back to 0h. Each wedge measures about 15 degrees across, and 15 times 24 equals 360 degrees, a full circle.

When moving across the RA, you’re essentially moving along the east-west axis. The RA coordinate numbers will increase from west to east from 0-24, and then reset back to 0 and repeat. If Star-A has an RA hour coordinate of of 5h and Star-B’s RA hour is 7h, then “A” always rises and sets before “B.” If “C” is at 22h and “D” is at 2h, then “C” will always rise and set before D.

This applies for the Sun, Moon, and planets too, even as their positions always change by day. For example, if the Moon’s RA hour is 7h and the Sun is at 10h… then on that 24 hour period, the Moon rises and sets before the Sun does. If the Sun and Moon, or any fixed celestial objects have an RA difference of 12 hours, then they’re on opposite sides of the sky, and within that 24 hour period, one rises just as the other sets.

See how that works?!


Many equatorial telescopes are equipped with setting circles, one for RA and one for Dec, and if you learn how to use them, then you can get your telescope accurately pointed at an object with ease without too much search. Even if the circles are not 100% accurate, they at least help you point at the general area the desired object is at! This article will show you how to use them properly.


Declination – The Sky’s Latitude

Declination is essentially your north-south movement. Unlike Right Ascension which is divided into 24 hourly sections, Declination is more like Earth’s latitude system, measured in degrees, where the celestial equator has a declination of 0° while the north celestial pole is at +90° and the south celestial pole is at -90°. It also uses minutes (‘) and seconds (“) to measure the precise locations – though you should note their markings to differentiate them from RA coordinates (° ‘ ” vs   h m s).

Remember that 60 arc-seconds (“) is one arc-minute (‘), and 60 arc-minutes is one degree (°).

Sirius’ declination is −16° 42′ 58.0171″. That means the star is located about 16 degrees below the celestial equator. The top right star of Orion’s Belt, Mintaka, is right at the celestial equator (technically 17′ south of it).

Polaris declination is currently +89° 21″ 7”. That means the North Star is not perfectly at the North Celestial Pole, and there is less than a degree of separation between the star and the pole itself.

Your latitude on Earth correlating with the sky’s declination coordinates determines how long certain stars will appear in the sky over your location, and if you can see them at all.

  • Observers at any of the Earth’s poles cannot see any stars below their respective celestial equator.
    • Using Orion as an example, Betelgeuse can be observed from the north pole but not the south, and vise versa with Rigel, while the Belt Stars (located near the celestial equator) are always at or below the horizon.
  • If a certain star’s declination is greater than 90 minus your latitude, than the star never sets above your location. You can try it on any star in your respective celestial hemisphere.
    • That is why most northern observers always see the Big Dipper and Cassiopeia circle around the North Star throughout the night all year.
      • For observers in Los Angeles and locations south of the 34th parallel, the Big Dipper actually at or below the northern horizon, hence why it’s difficult to spot during the Autumn months when at its lowest.
  • Canopus, the second brightest star in the night sky, has a declination of -52°, never sets in locations south of 37° 18′ and  is theoretically limited to locations below the northern 38th parallel.
    • From Los Angeles, Canopus is only in the sky for a few hours, and never gets higher than 3 degrees above the horizon – hence I’ve only seen it when the conditions are good enough.
  • Alpha Centauri never rises north of 29° N Latitude, and the entire Southern Cross cannot be seen unless you’re south of 25° N Latitude. Better plan that trip to Cancun or Hawaii!

 

Using Your Hands to Measure Degrees

While your hands are not true accurate measurements, they give you a good idea how long a certain number of degrees is in the sky, that way if someone tells you for example “{Object A} is x number of degrees away from {Object B},” you have a general idea.

hand-degrees

Remember that 60 arc-seconds(“) is one arc-minute (‘), and 60-arc minutes is one degree. The entire celestial sphere, including what you don’t see below the horizon, is 360.°

  • Depending on the focal length, a telescope at low power typically covers an area one degree in diameter.
  • A full moon is about 31′ across, or about half a degree.
  • The Sun moves east to west 15° every hour, no matter where you are on Earth!

Finding the Celestial Equator 

Besides finding your celestial pole, it’s also important to find the celestial equator, as some telescopes require alignment with it.

 

As discussed in The Basics of Navigating the Sky, the higher you are in latitude above the equator, the higher the respective celestial pole is in the sky. For example, Los Angeles is at around 34°N latitude. As such, Polaris, the north star is 34° above the northern horizon.

It’s a simple right angle triangle between you, the north celestial pole (where the declination is 90°), and the celestial equator (where the declination is 0°) on the opposite side of the sky. Therefore, the celestial equator over Los Angeles is 34° above the southern horizon.

CS.equator2.t

Notable constellations that the celestial equator crosses are Orion, Virgo, Aquila, Aquarius, and Cetus.

The Ecliptic – The Sun’s Path Across the Sky

The apparent path of the Sun’s motion on the celestial sphere as seen from Earth is called the ecliptic. The ecliptic plane is tilted about 23.45° with respect to the celestial equator since the Earth’s spin axis is tilted 23.45° with respect to its orbit around the Sun. The Sun’s declination over time changes, as it’s at its highest during the Summer months, and lowest during the Winter months. During the Spring and Autumn months, the Sun’s position is at or near the celestial equator.

What-is-the-Ecliptic

The following diagram shows how the Sun’s path changes throughout the year. During Summer Solstice, the Sun is highest, and rises more in a northeast direction while setting in a northwest direction. It also travels across more sky during the Summer, meaning longer days. During Winter Solstice, the opposite is in effect; rise in southeast, set in southwest, and less sky to travel across meaning shorter days. Only around Spring and Autumn Equinox does the Sun rise due east and set due west.

Sunpath2
Equinox days are represented by the blue line.

In the Northern Hemisphere, the Sun, Moon, and planets trace paths along the southern portions of the sky, and in the Southern Hemisphere, it’s the opposite, they appear to travel across the northern sector of the sky.

The line where the Sun, and any other celestial object has reached it’s highest in the sky, is called the Meridian line. If a star app lists a certain time when an object “transits” the sky, that means it’s listing the time when it crosses the meridian line.

The family of constellations where the ecliptic crosses are known as the Zodiac, and of course, this is where “star signs”or :star dates” come from… but thanks to this post, we know they are not the correct dates anymore. 

The ecliptic is also where you can find the planets and other Solar System objects, since most of them orbit along the same plane as the Sun.

Each planet’s orbits give them characteristics across the sky too! Their visibility is always dependent on their positions relative to the Sun and the constellations they’re in front of.

From Earth’s perspective, Mercury and Venus always appear to shift away from one side of the Sun, move back towards it, and then re-emerge on the other side. If either planet is east, or “left” of the Sun from Earth’s perspective, it’s visible in the evening, and vice versa on the other side. Mercury never gets further than 28° from the Sun, while Venus gets no further than 47°. This due to their inferior orbits.

The planets with superior orbits will always appear to shift their positions west to east each day, save for when they are in apparent retrograde. Their individual orbits are the cause of how fast their apparent shifts are. thus the longer their orbits, the longer they spend in front of a zodiac constellation.

Mars has a west-east shift of about one degree every 32-36 hours or so. It can spend as little as a couple weeks to just a few months in a constellation before moving on to the next.

Jupiter shifts a degree in about 97 hours, or just over 4 Earth days. As fast as that is, it can spend about a year in front of a constellation thanks to retrograde motion temporarily halting and reversing the shift. For 2020, it spends most of the year in front of Sagittarius before entering Capricorn just before 2021 begins. By April 2021, it’ll start shifting into Aquarius. 

Saturn shifts a degree every on average every 13-16 days . It usually spends about 2-3 years in front of a constellation. Like Jupiter, it is also in front of Sagittarius for 2020, and also begins 2021 in front of Capricorn.

Uranus shifts a degree every 28 days. But due to its 84 year orbit and its apparent retrograde motion every year, it can spend 5-7 years in front of a constellation. It’ll be in front of Aries from 2019 until 2024.

Neptune shifts a degree every 46 days, and the 165 year orbit ensures a much slower switch. Neptune has been in front of Aquarius since 2010, and won’t begin to be in front of Capricorn until 2022.

Just remember that these concepts are mastered with repeated observations. You too will always know where the planets are, and navigate with ease!

 

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