Altitude
0° is the theoretical horizon, 90° the zenith. A low object is seen through more atmosphere. Your personal threshold helps account for an obstructed horizon.
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The right time.
The right horizon.
Choose your location, target and session start. Explore the following 24 hours to prepare your observation.
110 Messier objectsMoon and 7 planetsFree · No account
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Altitude at the start
Direction at the start0° north · 90° east · 180° south · 270° west
Moon illumination at the startA bright Moon can hinder deep-sky observing.
A night in perspective
Gold: your target · Blue dashed line: the Moon · Green line: minimum altitude. The darkest bands indicate astronomical night.
Altitudes below −30° are clipped to the chart’s lower edge. The Moon and its curve overlap when you choose the Moon as your target.
Deep-sky windows
This dark-night filter is mainly useful for deep-sky observing. The Moon and planets can also be observed in twilight: check their altitude and times.
Times to remember
Events between the start and end of your 24-hour session, displayed in your time zone. Their order follows the phenomena, not chronology.
| Event | Local date and time |
|---|
“No event” means that no crossing occurs within this interval. An object may stay above or below the horizon, or may not rise or set during these 24 hours. A meridian transit may occur below the horizon.
The Moon and planets at the start
A position above the horizon does not guarantee that an object is discernible. Check darkness, its proximity to the Sun, the weather and your equipment.
| Celestial object | Altitude | Azimuth | Horizon |
|---|
View the hour-by-hour table
| Date and time | Target | Moon | Sky light |
|---|
Reading the sky with the right guides
Twilight
The Sun’s centre passes −6° for civil twilight, −12° for nautical twilight and −18° for astronomical twilight. In summer or near the poles, astronomical night may not occur.
The actual evening
Clouds, haze, turbulence, light pollution and obstacles are not calculated. Prepare several targets and check conditions on site.
Never point an instrument at the Sun without suitable, checked solar equipment. To prepare a solar observation, see the guidance in volume I.
Method, sources and limitations
Local calculation with Astronomy Engine 2.1.19 (Don Cross, MIT licence). Positions are topocentric, with precession, nutation, aberration and standard refraction for displayed altitudes. Twilight uses the geometric altitude of the Sun’s centre. Rise and set times use standard refraction and the upper limb for the Sun and Moon, on a theoretical clear horizon.
Messier object coordinates come from the portal catalogue, rounded to the minute of right ascension and arcminute, interpreted as J2000. The calculation is intended for planning observations, not precision pointing. The conventional distance of fixed objects serves only to make their parallax negligible.
Available period: 1900–2100. Time zones and clock changes rely on your browser’s database. A nonexistent local time is rejected; a repeated time requires choosing an occurrence. The window always contains 24 elapsed hours, even across a clock change. Times are rounded to the minute. Windows rely on the endpoints of 15-minute samples and may miss a very brief crossing.
Messier coordinates adapted from Steve Cimolino’s volume II, NASA HEASARC andOpenNGC, Mattia Verga. The adapted OpenNGC data, converted here to decimal values, remain licensed under CC BY-SA 4.0.