Understanding Maps

When you first try to use a star map, you’ll notice something confusing: there are two completely different coordinate systems, and they don’t work the same way at all. One system - Altitude-Azimuth (Alt-Az) - describes where to point based on your local horizon and compass directions. It’s intuitive and easy to understand. “Look 45 degrees up from the eastern horizon” makes immediate sense. The other system - Right Ascension and Declination (RA-Dec) - describes positions using celestial coordinates that seem abstract and disconnected from what you’re actually seeing. “Look at RA 5h 30m, Dec +22°” tells you nothing about where to point unless you already know what you’re doing.

So why do we use both? And if Alt-Az is simpler, why bother with RA-Dec at all? Here’s the short answer: Alt-Az coordinates are useful for telling you where to look right now, but they’re useless for mapping the sky permanently. Alt-Az maps are prepared for a given location and given time. They do not work all the time or everywhere. For example, a star map for New York City at 10 PM on January 1st is completely different from one for Los Angeles at 10 PM on July 1st. You need a new Alt-Az map for every location and every time you want to observe. RA-Dec coordinates work everywhere, for everyone, at any time. They’re universal.

This chapter explains both systems, why Alt-Az has fundamental limitations, and why RA-Dec became the standard for astronomical maps and catalogs. Once you understand the difference, reading star charts becomes much easier, and you’ll know which coordinate system to use for different tasks.

Altitude-Azimuth Maps

Altitude-Azimuth - usually shortened to Alt-Az - is the most intuitive way to describe where something is in the sky. It uses two measurements: how high up an object is (altitude) and which compass direction it’s in (azimuth). If someone tells you “Jupiter is 30 degrees above the southern horizon,” you know exactly where to look. You face south, tilt your head up about three fist-widths (remember, your fist at arm’s length covers roughly 10 degrees), and there it is. Alt-Az coordinates work the way your brain naturally thinks about the sky. You’re standing on the ground. There’s a horizon all around you. Objects are somewhere between that horizon and the point directly overhead. That’s it.

How Alt-Az Works

Altitude measures how high an object is above the horizon, in degrees.

The horizon is 0 degrees. The zenith (the point directly overhead) is 90 degrees. Anything between the horizon and zenith falls somewhere from 0 to 90 degrees. You can’t have negative altitude for visible objects - if something is below the horizon, you can’t see it. Some systems use negative altitudes to describe objects below the horizon (useful for calculating rise and set times), but for practical observing, altitude runs from 0 to 90 degrees.

Azimuth measures the compass direction, also in degrees. North is 0 degrees (or 360 degrees - same thing). East is 90 degrees. South is 180 degrees. West is 270 degrees. So if an object has azimuth 135 degrees, it’s halfway between east (90°) and south (180°) - that’s southeast. Together, altitude and azimuth pinpoint any object in the sky relative to your position.

Example: “Betelgeuse is at altitude 55°, azimuth 210°.” That tells you: face southwest (210° is between south and west), look up about halfway between the horizon and zenith (55° is a bit more than halfway to 90°), and you’ll find Betelgeuse. Simple. Intuitive. Easy to use.

Why Alt-Az Feels Natural

Alt-Az coordinates match how we experience the world. You’re always standing on a surface with a horizon around you. You naturally think in terms of “up” and compass directions. No abstract celestial spheres or invisible coordinate grids - just practical, real-world orientation. This is why Alt-Az mounts (like Dobsonians) are so popular for beginner telescopes. Move the telescope left-right to change azimuth. Move it up-down to change altitude. There’s no need to understand Earth’s rotation axis or celestial poles. Point and look. Alt-Az is also useful when giving someone verbal directions. If you’re outside with friends and want to show them Saturn, saying “look about 40 degrees up in the southwest” works instantly. Saying “look at RA 21h 30m, Dec -15°” means nothing to them.

The Fatal Flaw: Alt-Az Coordinates Change Constantly

Here’s the problem: Alt-Az coordinates are local and temporary. They only work for one specific observer at one specific time. Earth rotates. As it does, objects in the sky appear to move. A star that’s at altitude 30°, azimuth 90° right now will be at a completely different altitude and azimuth an hour from now. By tomorrow night at the same time, it’ll be in yet another position. Even worse, two observers standing in different locations see different Alt-Az coordinates for the same object at the same moment. Let’s say you’re in Mumbai and I’m in Chennai, and we’re both looking at the Moon at exactly 9:00 PM. You might see the Moon at altitude 50°, azimuth 120°. I might see it at altitude 45°, azimuth 115°. We’re looking at the same Moon at the same time, but our Alt-Az coordinates are different because we’re in different locations.

This makes Alt-Az coordinates useless for creating permanent maps or catalogs. You can’t publish a book that says “the Andromeda Galaxy is at altitude 60°, azimuth 45°” because that’s only true for one specific person at one specific time in one specific place. An hour later, it’s wrong. A different city, it’s wrong. A different date, it’s wrong.

Why Alt-Az Maps Are Rare

You won’t find any star atlases or catalogs using Alt-Az coordinates for exactly this reason. They’d be useless the moment you printed them. Imagine trying to create a map: “Orion’s belt stars are at altitude 40°, azimuth 180° on December 15th at 9 PM in New Delhi.” Great - but that information is worthless if you’re in Bangalore, or if it’s 10 PM, or if it’s December 16th. You’d need thousands of different maps for every combination of location, date, and time. Alt-Az coordinates are perfect for right now, where you are, but terrible for anything permanent or universal.

Where Alt-Az Is Still Useful

Despite its limitations, Alt-Az has practical uses: Planetarium apps and software use Alt-Az all the time because they’re calculating positions in real time for your specific location. Open an app like Stellarium or SkySafari, and it shows you the current sky with Alt-Az overlay. The app knows where you are and what time it is, so it can compute the correct Alt-Az coordinates for that exact moment.

Verbal directions work best in Alt-Az. “Look 30 degrees above the eastern horizon” is clearer than “look at RA 6h, Dec +20°.” Simple telescope mounts use Alt-Az because it’s mechanically straightforward. Move left-right for azimuth. Move up-down for altitude. Done. Finding your bearings when you first step outside uses Alt-Az thinking naturally. You orient yourself to the horizon and compass directions before you start identifying constellations or deep-sky objects.

Alt-Az for Tracking Objects

Here’s another limitation: tracking objects with Alt-Az mounts is awkward. As Earth rotates, stars appear to move in curved arcs across the sky. To keep a star centered in your telescope, you have to constantly adjust both altitude and azimuth simultaneously. The adjustments aren’t uniform - sometimes you adjust more in azimuth, sometimes more in altitude, depending on where the object is. Near the zenith, this gets especially messy. An object passing directly overhead requires rapid changes in azimuth but minimal changes in altitude. It’s mechanically awkward and difficult to track smoothly. Computerized Alt-Az mounts solve this by calculating the required adjustments automatically, but manual Alt-Az tracking is frustrating compared to equatorial mounts, which only require adjustment along one axis.

The Bottom Line

Alt-Az coordinates are intuitive, easy to understand, and useful for immediate, real-time observation. They’re how you naturally think about the sky when you’re standing outside looking up. But they’re local and temporary. They only work for you, right now, where you are. They can’t be used to create permanent maps or universal catalogs. They can’t describe where an object “is” in any lasting sense. That’s why astronomy needs a different system - one that describes positions in a way that works for everyone, everywhere, at any time. That system is Right Ascension and Declination.

Right Ascension and Declination Maps

Right Ascension and Declination - RA and Dec - solve the problem that Alt-Az can’t: they provide coordinates that work for everyone, everywhere, at any time. These coordinates don’t describe where to point your telescope right now. They describe where an object sits on the celestial sphere - that imaginary ball surrounding Earth that we project the sky onto. Once you know an object’s RA and Dec, you know its permanent position in the sky, regardless of when or where you’re observing.

This is why every star catalog, every deep-sky atlas, and every astronomical database uses RA and Dec. They’re universal. Betelgeuse has the same RA and Dec whether you’re in Chennai or Chicago, whether it’s January or July, whether it’s 9 PM or 3 AM. RA and Dec are more abstract than Alt-Az. They take longer to understand. But once you get them, navigating the sky becomes far easier.

How RA and Dec Work

RA and Dec are the celestial equivalent of longitude and latitude on Earth. Just like longitude and latitude pinpoint locations on Earth’s surface, RA and Dec pinpoint positions on the celestial sphere.

Declination (Dec) is the easier one - it works exactly like latitude. The celestial equator sits at 0° declination. It’s Earth’s equator projected outward onto the celestial sphere. Everything north of the celestial equator has positive declination. Everything south has negative declination. The north celestial pole is at +90° Dec. The south celestial pole is at -90° Dec. Betelgeuse, sitting north of the celestial equator, has a declination of about +7°. The Southern Cross, well below the celestial equator, has declination around -60°. Simple. Declination tells you how far north or south an object is from the celestial equator.

Right Ascension (RA) is trickier. It’s the celestial equivalent of longitude, but instead of degrees, it’s measured in hours, minutes, and seconds. Why hours? Because the sky rotates. Earth spins once every 24 hours (actually 23 hours 56 minutes, but close enough). As it does, the celestial sphere appears to rotate overhead, completing one full cycle in 24 hours. By dividing the celestial equator into 24 hours, each hour of RA corresponds to 15 degrees of arc (360° ÷ 24 = 15° per hour). RA runs from 0h to 24h as you move eastward around the celestial equator. The zero degree longitude line on Earth is the Prime Meridian in Greenwich, England. This was chosen arbitrarily with no scientific reason. Similarly, the zero point for RA is defined by a specific point in the sky: the vernal equinox. The zero point - 0h RA - is defined by the vernal equinox, the point where the Sun crosses the celestial equator moving northward each March. From there, RA increases: 1h, 2h, 3h… up to 23h 59m 59s, then back to 0h.

Why RA Uses Hours Instead of Degrees

This confuses beginners, but it’s actually practical. The sky rotates at a constant rate - 15 degrees per hour. If an object has RA 6h and another has RA 9h, they’re separated by 3 hours of RA, which equals 45 degrees of sky (3 × 15 = 45). More importantly, the time-based system directly relates to when objects are visible. If your Local Sidereal Time (LST) is 6h, then objects with RA 6h are crossing your meridian right now. Objects with RA 3h crossed three hours ago. Objects with RA 9h will cross three hours from now. Using hours makes the connection between position and timing intuitive once you understand LST. Degrees would work mathematically, but hours sync better with how the sky actually moves. Refer to the chapter on Local Sidereal Time for a deeper explanation of how RA in hours ties directly to when objects are visible in your sky.

RA and Dec Are Fixed (Mostly)

Here’s the key advantage: an object’s RA and Dec don’t change as Earth rotates or as you move to a different location. Betelgeuse is at roughly RA 5h 55m, Dec +7° 24’. That’s true whether you’re in India, Australia, or Canada. It’s true at midnight or at dawn. It’s true in winter or summer (though it might be below your horizon depending on the season and time).

The coordinates are fixed to the celestial sphere. The sphere appears to rotate as Earth spins, but the coordinates stay constant. This lets you create star atlases, catalogs, and databases that work universally. There’s a small caveat: RA and Dec do change very slowly over decades and centuries due to precession - Earth’s axis wobbles like a spinning top, causing the celestial poles to shift. But this happens so gradually that for practical observing, you can ignore it. Star catalogs specify an “epoch” (like J2000.0 or J2025.0) to account for precession, but unless you’re doing precision work or using very old maps, it doesn’t matter.

Reading RA and Dec Coordinates

When you see coordinates written out, they look like this: Andromeda Galaxy (M31): RA 0h 42m 44s, Dec +41° 16’ 9”

Let’s break that down:

RA 0h 42m 44s means 0 hours, 42 minutes, 44 seconds of Right Ascension. Dec +41° 16’ 9” means +41 degrees, 16 arcminutes, 9 arcseconds of Declination.

The plus sign indicates it’s north of the celestial equator. A minus sign would mean south. For casual observing, you don’t need the precision of seconds. Rounding to RA 0h 43m, Dec +41° is close enough.

Using RA and Dec on Star Charts

Star charts are covered with a grid showing RA and Dec lines. RA lines run vertically (north-south), labeled in hours across the top or bottom of the chart. Dec lines run horizontally (east-west), labeled in degrees along the sides. To find an object:

Locate the RA hour along the bottom or top edge of the chart. Locate the Dec degree along the left or right edge. Follow those lines inward until they intersect - that’s your object.

For example, if you’re looking for something at RA 8h, Dec +30°, find where the 8h RA line crosses the +30° Dec line. The object will be at or near that intersection. Most detailed finder charts cover small regions of sky - maybe 5 or 10 degrees across - with finer subdivisions. You’ll see RA marked in 10-minute or even 1-minute increments and Dec marked in 1-degree or smaller steps. This precision helps you locate faint objects near brighter reference stars.

Converting RA/Dec to Where You Actually Look

Here’s the part that trips people up: RA and Dec tell you where an object is on the celestial sphere, but they don’t directly tell you where to point your telescope. To find an object using RA and Dec, you need to know:

Is the object above your horizon right now? That depends on your latitude, the time of night, and the date. An object might have RA 12h, Dec +20°, but if that part of the celestial sphere is currently below your horizon, you won’t see it.

Where should you point? This depends on your Local Sidereal Time (LST) and the object’s coordinates. If your LST equals the object’s RA, the object is crossing your meridian - the imaginary line running from north to south through your zenith. If your LST is higher than the object’s RA, the object is in the western sky. If lower, it’s in the eastern sky.

This is where planetarium apps and star charts help. Apps calculate what’s visible based on your location and time. Star charts show you the sky for specific dates and times. Both translate RA/Dec coordinates into practical “where to look” information.

Why RA/Dec Is Better for Maps

Imagine trying to create a star atlas using Alt-Az. You’d need separate maps for every city, every hour of the night, every day of the year. Thousands of maps, all slightly different, all outdated the moment conditions change. With RA/Dec, you create one map. It works everywhere. Forever (ignoring precession over centuries). The Orion Nebula is at RA 5h 35m, Dec -5° 23’. That’s true in Mumbai, New York, Tokyo, and Sydney. A printed atlas can list that coordinate, and any observer anywhere can find it - as long as they know their LST and can translate RA/Dec into local viewing directions.

This universality is why professional astronomy uses RA/Dec exclusively. When scientists publish a discovery, they give RA and Dec coordinates. When telescope databases catalog objects, they use RA and Dec. When you look up an object in an app or website, you’ll see RA and Dec listed.

RA/Dec for Telescope Tracking

Equatorial telescope mounts are designed around RA and Dec. Once you align an equatorial mount to the celestial pole, one axis tracks RA and the other tracks Dec. As Earth rotates, objects move westward along lines of constant Dec. To keep an object centered, you only adjust the RA axis. The Dec axis stays fixed (assuming the object isn’t drifting north or south, which it won’t over short periods). This makes tracking smooth and simple - especially with a motor drive. The motor rotates the RA axis at the same rate Earth rotates, keeping the object perfectly centered without any manual adjustment. Alt-Az mounts require constant adjustment on both axes to track objects. Equatorial mounts aligned to RA/Dec only require adjustment on one axis. For visual observing, this is convenient. For astrophotography, it’s essential.

Learning to Think in RA/Dec

RA and Dec feel abstract at first because they’re disconnected from your immediate experience. You can’t look up and instantly know which RA is overhead the way you can estimate altitude by feel. But with practice, RA and Dec start to make intuitive sense. You learn that certain constellations live at certain RA ranges. Orion is around 5h to 6h RA. Sagittarius is around 18h to 19h RA. The summer Milky Way spans roughly 18h to 21h RA. You start connecting RA to seasons. Objects around 6h RA are visible in winter evenings. Objects around 18h RA are visible in summer evenings. You internalize the patterns. Eventually, someone can tell you an object is at RA 12h, Dec +15°, and you’ll immediately know: “That’s in the spring evening sky, probably somewhere in Virgo or Leo.” It takes time, but it’s worth it. Once RA and Dec click, the entire sky becomes navigable.

The Bottom Line

RA and Dec are abstract, but they’re universal. They describe permanent positions on the celestial sphere that don’t change with time, location, or Earth’s rotation. Alt-Az tells you where to look right now. RA and Dec tell you where an object actually is in the cosmic coordinate system. Both have their uses. Alt-Az is intuitive for immediate observation. RA and Dec are essential for maps, catalogs, databases, and understanding the structure of the sky. Learn both. Use Alt-Az when you’re outside pointing at things. Use RA and Dec when you’re planning observations, reading star charts, or looking up objects in catalogs. Together, they give you complete control over navigating the night sky.