Where is it? Celestial coordinates explained

Summary

This video script explores various coordinate systems, starting with the Earth's latitude and longitude, which use the equator and prime meridian as references. It then delves into celestial coordinates, highlighting the equatorial system with right ascension and declination, and the historical ecliptic system. The script also introduces the horizontal coordinate system (Alt-Az), which is based on local observations relative to the horizon and north. The presenter, Nico Carver, simplifies complex astronomical concepts and mentions tools like GO-TO systems and planetarium apps that aid in understanding these systems, promising a future discussion on the apparent size of celestial objects.

Takeaways

• 📏 A typical coordinate system uses reference planes to define positions, like latitude and longitude on Earth.
• 🌐 Latitude measures the angular distance north or south from the equator, while longitude measures east or west from the prime meridian in Greenwich, England.
• 🌟 In celestial coordinates, the equatorial system uses the Earth's equator as a reference plane extended into space.
• 🕰 Right ascension, part of equatorial coordinates, is measured eastward from the sun's position at the vernal equinox and is often expressed in hours, minutes, and seconds.
• 🌡 Declination in the equatorial system is similar to latitude, measuring up or down from the celestial equator, and is expressed in degrees.
• 🔭 The equatorial mount's 'GPS', or GO-TO system, uses location and time data to automatically point a telescope at celestial objects.
• 🌄 The horizontal coordinate system, or Alt-Az, uses the observer's local north and horizon as reference planes, varying with time and location.
• 📱 Planetarium software and apps can convert equatorial coordinates to horizontal coordinates, simplifying astronomical observations.
• 📖 The book 'Longitude' by Dava Sobel provides a detailed history of why Greenwich was chosen as the prime meridian reference.
• 🌌 The vernal equinox's significance in astronomy is its use as a fixed point for measuring right ascension in the celestial coordinate system.

Q & A

What are the two reference planes in a typical coordinate system?

In a typical coordinate system, the two reference planes are used to define any location as the distance from these planes.

What is the significance of the equator and the prime meridian in the context of latitude and longitude?

The equator serves as the reference plane for measuring latitude, which is the angular distance north or south from the equator. The prime meridian in Greenwich, England, is the reference for measuring longitude, which is the angular distance east or west from Greenwich.

Why do we measure longitude from the Royal Observatory in Greenwich, England?

The script does not provide the reason in this context, but it suggests reading 'Longitude' by Dava Sobel for a detailed explanation.

What is the ecliptic and how does it relate to the zodiac constellations?

The ecliptic is the path that the sun appears to follow over the course of a year, and it is the basis for the positions of the zodiac constellations.

How does the equatorial coordinate system differ from the ecliptic system?

The equatorial coordinate system uses the Earth's equator as a reference plane extended into space, unlike the ecliptic system which is based on the sun's path.

What are the two components of equatorial coordinates for the night sky?

The two components are right ascension, measured eastward from the sun's position on the vernal equinox, and declination, measured from the Earth's equator projected into space.

Why is right ascension commonly measured in hours, minutes, and seconds instead of degrees?

Right ascension is measured in hours, minutes, and seconds to divide the celestial sphere into 24 parts, analogous to the hours on a clock, rather than 360 degrees.

What is a GO-TO system in the context of equatorial mounts for telescopes?

A GO-TO system is an automated feature that uses the observer's location, time, and date to calculate and point the telescope to specific celestial objects.

What is the horizontal coordinate system, also known as Alt-Az, and how does it differ from equatorial coordinates?

The horizontal coordinate system, or Alt-Az, uses the observer's local north and the horizon as reference planes, providing coordinates that are specific to the time and location of the observation. It differs from equatorial coordinates which are constant regardless of the observer's location and time.

Why are horizontal coordinates (Alt-Az) easier to understand for observers?

Horizontal coordinates are easier to understand because they relate directly to the observer's local environment, with altitude indicating height above the horizon and azimuth indicating direction from north.

How can the apparent position of celestial objects change when switching from equatorial to horizontal coordinates?

The apparent position of celestial objects can change when switching from equatorial to horizontal coordinates because the latter is dependent on the observer's location and the time of observation, while the former is a fixed reference frame.

Outlines

00:00

🌐 Understanding Coordinate Systems

This paragraph introduces the concept of coordinate systems, explaining that any location can be defined by distances from reference planes. The example of latitude and longitude is used, with the equator and the prime meridian in Greenwich, England, serving as reference planes. Latitude measures the angular distance north or south from the equator, while longitude measures east or west from Greenwich. The speaker's current coordinates are given as 42 degrees north and 71 degrees west, illustrating the concept. The paragraph also touches on the historical choice of Greenwich as the starting point for longitude measurements, suggesting further reading in Dava Sobel's book 'Longitude'. The discussion then shifts to celestial coordinate systems, highlighting the ecliptic and equatorial systems. The equatorial system uses the Earth's equator as a reference plane and introduces the concept of right ascension and declination, which are used to locate stars in the night sky. Right ascension is measured eastward from the sun's position during the vernal equinox, while declination is measured from the Earth's equator projected into space. The paragraph concludes with a mention of the equatorial coordinate system's practical application in astronomy, including the use of star charts and equatorial mounts for telescopes.

Keywords

💡Coordinate System

A coordinate system is a framework that uses one or more reference planes to define positions in a space. In the context of the video, it's used to explain how locations on Earth and in the sky are pinpointed. The video discusses different types of coordinate systems, including the geographic coordinate system using latitude and longitude, and celestial coordinate systems used in astronomy.

💡Latitude

Latitude refers to the angular distance of a location on Earth north or south from the equator. It is a fundamental component of the geographic coordinate system. The video uses latitude to explain how one's position on Earth is determined relative to the equator, with the speaker giving their own coordinates as an example.

💡Longitude

Longitude is the angular distance of a location on Earth east or west from the prime meridian, which is a line that runs through Greenwich, England. It works in conjunction with latitude to specify precise locations on the Earth's surface. The video explains the concept of longitude and its historical significance.

💡Equatorial Coordinate System

The equatorial coordinate system is a celestial coordinate system that uses the Earth's equator as a reference plane when mapping positions of celestial objects. The video explains that this system is more commonly used today than the ecliptic coordinate system and consists of two parts: right ascension and declination.

💡Right Ascension

Right ascension is a coordinate in the equatorial coordinate system that measures the angular distance of a celestial object eastward from the vernal equinox point along the celestial equator. It is analogous to longitude on Earth but is measured in hours, minutes, and seconds instead of degrees. The video clarifies that right ascension is a time-based measurement, dividing the celestial sphere into 24 hours.

💡Declination

Declination in the equatorial coordinate system is the angular distance of a celestial object north or south from the celestial equator, similar to how latitude works on Earth. It is measured in degrees, with the equator being the 0-degree reference line. The video uses declination to describe how stars are positioned above or below the celestial equator.

💡Celestial Sphere

The celestial sphere is an imaginary sphere with the Earth at its center, upon which all celestial objects appear to be projected. The video mentions the celestial sphere in the context of dividing it into degrees for declination and hours for right ascension, illustrating how astronomers map the night sky.

💡Vernal Equinox

The vernal equinox is one of the two points in the year when the Sun crosses the celestial equator, marking the beginning of spring in the Northern Hemisphere. In the video, the vernal equinox is significant as it is the reference point for measuring right ascension in the equatorial coordinate system.

💡Horizontal Coordinate System

Also known as Alt-Az (altitude-azimuth), the horizontal coordinate system is a local coordinate system that uses the observer's local horizon and the cardinal directions as reference planes. The video explains that this system is easy to understand and is based on the observer's current location and time, making it context-specific.

💡Altitude and Azimuth

Altitude and azimuth are the two coordinates used in the horizontal coordinate system. Altitude is the angle between the horizon and the object, while azimuth is the compass direction from which the object is seen. The video uses these terms to describe how objects in the sky are located relative to an observer's position.

💡Zenith

The zenith is the point in the sky directly above an observer. It is used in the context of the horizontal coordinate system to describe the highest point an object can reach in the sky, which is at an altitude of 90 degrees. The video mentions zenith to help understand the concept of altitude.

Highlights

A typical coordinate system uses at least two reference planes to define any location.

Latitude/longitude uses the equator and the prime meridian in Greenwich as reference planes.

Latitude measures angular distance north or south from the equator.

Longitude measures angular distance east or west from Greenwich.

The reason for starting longitude measurement from Greenwich is detailed in Dava Sobel's book 'Longitude'.

The ecliptic, the sun's path throughout the year, is an ancient celestial coordinate system reference plane.

The equatorial coordinate system uses the earth's equator as a reference plane extended into space.

The celestial coordinate system's second reference plane is based on the sun's position at the vernal equinox.

Right ascension in equatorial coordinates is measured eastward from the vernal equinox point.

Declination in equatorial coordinates is measured from the earth's equator projected into space.

Declination is measured in degrees, similar to latitude.

Right ascension is commonly measured in hours, minutes, and seconds, dividing the celestial sphere into 24 hours like a clock.

Equatorial mounts with GO-TO systems automate telescope pointing based on location, time, and date.

Horizontal coordinate system (Alt-Az) uses the observer's local north and horizon as reference planes.

Alt-Az coordinates change with time of day but are easier to understand for immediate observation.

Azimuth in Alt-Az is the angular distance from north, with 90 degrees indicating due east.

Altitude in Alt-Az is the angular distance from the horizon, with 90 degrees indicating straight up at the zenith.

Modern software like Stellarium can convert equatorial to horizontal coordinates automatically.

Upcoming discussion will cover the apparent size of space objects and how field of view affects deep sky object viewing.

Transcripts

In a typical coordinate system you have at least two reference planes and then any location

can be explained as the distance from those planes.

So, for example, for latitude/longitude, the two reference planes are the equator and the

prime meridian in Greenwich, england.

Latitude is your angular distance either north or south from the equator and longitude is

your angular distance east or west from Greenwich.

So when I say my coordinates right now are 42 degrees north by 71 degrees west.

That means I’m 42 degrees north of the equator, and 71 degrees west of the British Royal Observatory

in Greenwich, England.

Now you may have never considered why do we start measuring Longitude from that specific

place in Greenwich.

I won’t answer that in this video, but I will point you to a great book by Dava Sobel

called Longitude for the whole story there.

Now, let’s move on to the stars in what we call a celestial coordinate system.

There are lots of possible reference planes for making a celestial coordinate system and

one of the earliest used by ancient people was the ecliptic which is the path that the

sun moves throughout the year and is where the zodiac constellations come from.

The more common coordinate system today though is equatorial which as the name suggests uses

the earth’s equator as a reference plane, basically just imagine the equator of the

earth going out into space forever.

The other reference plane is a bit more complex to wrap your head around as it’s both a

time and a location.

The reason it has to be both a time and a location is we are both rotating around our

own axis, but also rotating around the sun, which is why there are different constellations

up in the summer compared to the winter, because we are literally facing a different part of

the milky way.

So the other reference plane is where the sun is located in the night sky at the time

of the vernal equinox in March, we then measure where the stars are located eastward from

that plane and call this right ascension.

So to review: the two parts that make up equatorial coordinates for the night sky are first right

ascension which is measured eastward from where the sun is in the night sky on the vernal

equinox, and then declination which is measured from the earth’s equator projected out into

space, up or down, with the equator being 0 degrees.

Now to make this just a bit more confusing.

Declination like latitude is measured in degrees.

Right ascension could be measured in degrees and sometimes is, but more commonly is measured

in hours, minutes and seconds.

So instead of breaking the celestial sphere into 360 parts we call degrees, we break it

into 24 parts we call hours like a clock.

Okay, so that’s the equatorial coordinate system, you can use a star chart and what

are called setting circles on an equatorial mount if you want to go full manual.

What I think of as the ‘GPS’ of equatorial mounts is a GO-TO system where you tell the

system where on earth you are located and the precise time and date, and then it translates

where the stars should be in the sky, and automates the pointing of the telescope.

Now the last thing I want to talk about is another kind of coordinate system we use all

the time in astronomy called a horizontal coordinate system, also known simply as Alt-Az.

Which is short for altitude and azimuth.

With alt-az the two reference planes are North and the horizon at the actual time and location

of where you are observing.

It’s called a horizontal coordinate system, but another way I’d think of it is just

a local coordinate system that’s just for that moment.

The coordinates don’t really mean anything outside of that context.

You can see if I change my time of day in stellarium, the alt/az keep changing, but

the Ra/dec stay consistent.

BUT the reason we use alt/az all the time is they are a lot more easy to understand.

For instance if I pull up Alberio here and see the azimuth is 90 degrees I know that

is due east, and if it’s altitude is 45 degrees I know that’s halfway up from the

horizon to the zenith.

The Zenith means straight up, which is 90 degrees.

Now there are some fancy formulas for converting equatorial coordinates to horizontal coordinates,

but luckily we live in an age where all of that info is included in free software like

stellarium or any number of free or low cost planetarium apps on your smartphone.

Next week I’ll be talking about the apparent size of objects in space, and how calculating

your field of view can tell you what kinds of deep sky objects will work best for your

gear.

Till then, this has been Nico Carver from Nebula Photos.

Clear skies!