When I go outside to observe the jewels of the night sky with my binoculars or telescope, I like to use a star chart, also known as a star map. It’s also possible to use an app on your mobile phone that represents a digital star chart. But I recommend beginners start with a traditional star chart because it’s simple to use. Most importantly… you’ll learn to find the celestial objects in the night sky without counting on software or mobile devices. These can sometimes let you down. Now let’s dive into what a star chart is and more about it.
What is a Star Chart?
A star chart, also known as a star map, is a map of the night sky that astronomers use to identify and locate astronomical objects such as stars, constellations, galaxies and planets. It is a fundamental tool in celestial navigation and has been used since ancient times to aid in the understanding and organization of the cosmos.
A star chart represents the positions of the stars as they appear from Earth. The chart is usually a two-dimensional projection of the celestial sphere, which is an imaginary sphere surrounding the Earth onto which all celestial bodies are projected.
There are also three-dimensional star charts like Stellarium, SkySafari, Cartes du Ciel and Google Sky Maps. These digital star charts you can install as an app on your mobile phone, tablet, laptop or PC.
Star charts are based on a combination of astronomical observations, scientific principles, and historical conventions.
Astronomers observe and record the positions of stars and other celestial objects. These observations are the primary data used in creating star charts.
How do you read a star chart?
Reading a star chart effectively is a skill that allows you to navigate and understand the night sky. A star chart is a map of the stars and constellations as seen from Earth.
Here’s a step-by-step guide on how to read one:
- Understanding the Layout:
- Orientation: Most star charts are circular, representing the dome of the night sky. The chart’s edge represents the horizon, while the center depicts the sky directly overhead.
- Coordinate System: Like Earth, the sky has a coordinate system. It includes:
- Right Ascension (RA): Celestial equivalent of longitude.
- Declination (Dec): Celestial equivalent of latitude.
- Scale: Some charts show the entire visible sky, while others focus on specific sections.
- Identifying Constellations and Stars:
- Constellations: These are patterns of stars, often named after mythological figures, animals, or objects. A star chart will show these patterns, helping you to identify them in the sky.
- Star Names and Brightness: Stars are often named (e.g., Sirius, Polaris) and are depicted in different sizes based on their brightness (magnitude).
- Using the Chart According to Time and Date:
- Star charts are often designed for specific dates and times. To use one, you need to know the current date and time.
- Some charts have a rotating disk (planisphere) that aligns the visible constellations with the time and date.
- Aligning the Chart with the Actual Sky:
- Direction: Determine which direction you are facing (North, South, East, West) and align the chart accordingly. You can use a compass if you’re unsure in which direction you are facing.
- Altitude and Azimuth: These are used to find the height of a star above the horizon and its direction.
- Locating Planets and the Moon:
- Unlike stars, planets and the Moon move relative to the background stars, so their positions change nightly. Specialized star charts or apps may be needed to locate them accurately. Because planets and the Moon change a lot, they are often not mapped on traditional two-dimensional star charts made from paper, cardboard or plastic. But you can always find the planets and the Moon on three-dimensional digital star charts or apps like Stellarium or SkySafari which are designed to track and predict their positions over time.
- Reading Special Features:
- Milky Way: Often shown as a cloudy band.
- Deep Sky Objects: Galaxies, nebulae, and star clusters may be marked.
- Using a Planisphere:
- A planisphere is a simple, adjustable star chart. Rotate the disk to match the date with the time, and it shows the stars visible at that moment. I like to use a planisphere when I’m observing or imaging with my binoculars so that I don’t have to drag or carry a lot of other stuff (other big books, laptop, tablet, etc.) outside.
Here is a photo of my planisphere where the disk is set on the 18. of January at 00:00 am (red triangle). The “1” below the “0” in the red triangle stands for 01:00 am DST (Summertime).
- Digital Star Charts:
- Modern digital star charts or apps can be extremely helpful. They often use GPS to display the exact star map for your location and time. Well-known digital star charts among astrophotographers are Stellarium, SkySafari and Cartes du Ciel.
When I’m outside doing astrophotography and want to search for a target to image, I like to use SkySafari or the SkyAtlas from ZWO. Most of the time, I’m using the ZWO ASIair v1 or ZWO ASIair Plus to photograph an object. The SkyAtlas from ZWO is included in the app that I use to image with my ZWO ASIair v1 or Plus. This makes it very convenient to search, frame and image my target of interest. The ZWO ASIair is an electronic device I use for telescope control and data acquisition. More about the ZWO ASIair in another article.
Tips for Effective Use:
- Practice: Familiarize yourself with the chart by identifying easy-to-find constellations and stars.
- Dark Environment: Use the chart in a dark environment to avoid light pollution. Allow your eyes to adapt to the dark for about 20-30 minutes before observing the stars for better visibility.
- Red Light: Use a red light to read the star chart or planisphere (see picture below). Red light preserves your night vision better than white or blue light.
Do star charts ever change?
Yes, star charts do change over time, reflecting the dynamic nature of the night sky. This change is primarily due to two main factors: the precession of the Earth’s axis and the proper motion of the stars. Let’s explore these in detail:
- Precession of the Earth’s Axis:
- Understand Precession: The Earth’s axis slowly wobbles, similar to a spinning top. This wobble, known as axial precession, completes a cycle approximately every 26,000 years.
- Impact on Star Charts: Due to precession, the position of the stars and constellations as seen from Earth gradually shifts. This shift is also called “axis shift”. Over centuries, this shift has become noticeable. For example, the North Star (Polaris) has not always been, nor will it always be, the star closest to the North Celestial Pole. Thousands of years ago, other stars like Thuban in the constellation Draco held that position (see below, at the left, star chart of the constellation Draco. The star Thuban is located about Northwest of the seven stars (Alkaid, Mizar, Alioth, Megrez, Phecda, Merak and Dubhe) that form the famous “Plough” or “Big Dipper” asterism of Ursa Major. In the future, the bright star Vega in the constellation Lyra will be the North Star (see star chart below, at the right) because the Earth’s axis is slowly wobbling away from Polaris. That will happen in 13,000 years when Earth’s axis will point toward the new North Star, the star Vega.
- Proper Motion of the Stars:
- Recognize Star Movement: Stars are not fixed in space; they move relative to each other. This movement, known as proper motion, is generally slow but accumulates over time.
- Consequences for Star Charts: Over long periods, the proper motion of stars can change their positions in the sky. This means that star charts need updating to accurately reflect these changes.
- Technological Advancements and Discoveries:
- Incorporate New Discoveries: As astronomy advances, we discover new celestial objects and phenomena. Star charts are updated to include these new findings.
- Utilize Improved Measurements: Technological advancements lead to more precise measurements of star positions and movements, necessitating updates to star charts.
- Changes in Earth’s Orbit:
- Account for Orbital Shifts: The Earth’s orbit around the Sun is not perfectly stable and undergoes slight changes over millennia. These changes can also affect the apparent positions of stars.
Where on the internet can you find free star charts?
You can find free star charts on various websites on the internet. Here are a few notable ones:
- freestarcharts.com: This website offers free astronomy charts, the latest night-sky news, in-depth constellation guides, and more. Visit freestarcharts.com for more information.
- In-The-Sky.org: They provide all-sky charts of the night sky, showing what stars and planets you’ll be able to see on any given day of the year. Check out their charts at In-The-Sky.org.
- Eyes on the Sky: This site offers free star charts, including a monthly all-sky star chart. You can find these charts at Eyes on the Sky.
What books with star charts are there on the market ?
There are several excellent books available in the market that feature star charts, catering to both amateur stargazers and seasoned astronomers. These books not only provide detailed star charts but also offer valuable information on celestial objects, constellations, and tips for stargazing. Here are some notable examples:
- “The Stars: A New Way to See Them” by H.A. Rey:
- User-Friendly Approach: This book, known for its clear and easy-to-understand star charts, is ideal for beginners. H.A. Rey, the author of the Curious George series, presents the constellations in a unique, simplified way that makes them easy to identify.
- “Turn Left at Orion: Hundreds of Night Sky Objects to See in a Home Telescope – and How to Find Them” by Guy Consolmagno and Dan M. Davis:
- Telescope Users’ Guide: This book is a fantastic resource for amateur astronomers who own telescopes. It provides detailed guides on how to locate and observe hundreds of celestial objects, accompanied by comprehensive star charts. I do not yet own this book myself. But according to several astrophotographers, this is a very good, clearly explained book and has been recommended to me by them.
- “NightWatch: A Practical Guide to Viewing the Universe” by Terence Dickinson:
- Comprehensive Guide: “NightWatch” is widely regarded as one of the best all-around guides for stargazing. It includes detailed star charts, information on the use of binoculars and telescopes, and tips on observing the night sky.
- “Sky & Telescope’s Pocket Sky Atlas” by Roger W. Sinnott:
- Compact and Detailed: This pocket atlas is a great portable guide for field use. It contains detailed charts and is particularly useful for intermediate to advanced stargazers. This one I own and can also recommend it to beginners. Just because it is in pocket size, you can easily take it anywhere. The star charts in this book are also easy to understand, once you know how to use them.
- “The Cambridge Star Atlas” by Wil Tirion:
- For Serious Stargazers: Known for its accuracy and detail, this star atlas is suitable for more experienced astronomers. It covers the entire sky in a series of detailed star charts. This is another book that has been recommended to me by many astrophotographers/astronomers because it is a very good book. Wil Tirion has a stellar reputation when it comes to star charts. He is known for his works the Sky Atlas 2000.0 first published in 1981, and Uranometria 2000.0, his most comprehensive work, from 1987. He is a Dutch uranographer (astro-cartographer) and he is probably the only person on Earth who earns a full living making star maps. However, he has never received any astronomical education and is thus self-taught.
- “The Observer’s Sky Atlas: With 50 Star Charts Covering the Entire Sky” by Erich Karkoschka:
- Observational Tool: This atlas is designed for practical use in the field and includes charts and data on various celestial phenomena, making it a valuable tool for observers.
What book with star charts can I recommend?
When I’m outside at my telescope and I want to use a traditional star chart instead of a digital star chart for observing and imaging, I like to use “Sky & Telescope’s Pocket Sky Atlas” by Roger W. Sinnott. It’s one I’ve been using already since 2017. I can highly recommend this book for the reasons mentioned here above in point 4.
Here is the link to the page of the website of Sky & Telescope, where it’s clearly explained how to use a star chart at the telescope:
https://skyandtelescope.org/astronomy-resources/using-a-map-at-the-telescope/
Conclusion
As a beginner, it can be very daunting or overwhelming to read a star chart but after a while, you get to know how to use it. When you start to know how to read a star chart, you will begin to notice that a star chart or star map is an invaluable addition to your astronomy books and accessories. Actively reading a star chart means engaging with its layout, aligning it with the current sky, identifying celestial objects, and using tools like planispheres or digital apps for accuracy. With regular practice, you’ll enhance your stargazing experience and deepen your understanding of the night sky.
Wisdom Wednesday: each Wednesday, I will highlight a concept in astrophotography.
I will keep it as short as possible and clarify it well with images and/or videos.
On Jan. 24 and Jan. 31, 2024, I was unable to post a new “Wisdom Wednesday” article due to certain circumstances.
