166 questions
All answers
From how fast the station moves to what telescope to buy first — one to three sentences each, with a number in every answer.
Questions
- How fast does the ISS travel? — About 27,600 km/h (7.66 km/s) — roughly 20 times a rifle bullet.
- How long is one ISS orbit? — About 92 minutes, so the crew sees 15–16 sunrises a day.
- Can I see the ISS with the naked eye? — Yes — it reaches −4 magnitude, brighter than Venus, if it is sunlit while your sky is dark.
- How long does an ISS pass last? — Usually 4–6 minutes; a high overhead pass can reach 7.
- Why are there no ISS passes this week? — Geometry: for several days it may cross your sky only in daylight or in Earth's shadow.
- How high is the ISS? — About 420 km, losing a few km a year to drag and reboosted periodically.
- Why is the ISS at 51.6° inclination? — Baikonur sits at 46°N; 51.6° is the cheapest plane Soyuz can reach from there.
- Why does the ISS always cross west to east? — It orbits with Earth's rotation, so the ground track runs roughly west to east.
- Can I see Tiangong from China? — Yes, more easily than the ISS: at 41.5° inclination most of China gets high passes.
- Tiangong or the ISS — which is brighter? — Tiangong is smaller (about 90 t vs 420 t) but still hits −2 magnitude overhead.
- How big is Tiangong? — About 100 t across three modules, with roughly 340 m³ of pressurised volume.
- When is the next rocket launch? — The launch board here, sorted by NET, with the snapshot time on every row.
- Why do launches keep slipping? — Weather (upper winds, cumulus), ground equipment and range safety cover most slips.
- What does NET mean? — No Earlier Than — the earliest possible time, and it usually moves.
- What is T-0? — The liftoff instant; the count runs from it and becomes T+ afterwards.
- What is a launch window? — The interval a launch may occur in; an ISS rendezvous window can be seconds long.
- Scrub versus delay? — A scrub cancels that attempt (maybe retry same day); a delay moves the date.
- What can I see during a launch? — At night the plume; stage separation usually comes 2–3 minutes in.
- When is the best time to stargaze? — Near new moon, clear, away from city light, and after astronomical twilight (Sun below −18°).
- How do I know if the sky is dark enough? — Use the dark-window time on a city page here; better than Bortle 5 to see the Milky Way.
- Can you see stars in a city? — Yes, but only bright ones: limiting magnitude near 4, so a few hundred at most.
- Why is a full moon bad for observing? — At about −12.7 magnitude it washes out faint objects; avoid ±5 days around full moon.
- Civil, nautical and astronomical twilight? — The Sun is 6°, 12° and 18° below the horizon; only the last means real darkness.
- Seeing or transparency — which matters? — Seeing for planets, transparency for deep sky.
- How many stars can you see by eye? — About 2,500–3,000 under a dark sky; often under 200 in a city.
- When can I see the Milky Way? — Summer in the north, under Bortle 5 or better, with no moonlight.
- How do I watch a meteor shower? — Wait for the radiant to rise, use naked eyes and a wide view — not a telescope.
- How do I tell a planet from a star? — Planets twinkle less, look warmer, and move against the stars over weeks.
- When can I see Venus? — Only near dawn or dusk, at up to 47° elongation — the brightest object after Sun and Moon.
- Can binoculars show Jupiter's moons? — Yes — steady 10×50s show all four Galilean moons, in different places each night.
- What telescope shows Saturn's rings? — About 30× separates them; a 70–90 mm refractor on a steady mount is enough.
- When is Mars brightest? — Around opposition, every 26 months, reaching −2.9 magnitude.
- What do I need to see lunar craters? — 10×50 binoculars show the big ones; 70 mm and up adds detail.
- What should my first telescope be? — Binoculars first (7×50 or 10×50), then a 6–8 inch Dobsonian.
- Binoculars or a telescope first? — Binoculars — wide field, easy to aim, and more to see in month one.
- What does a 1,000 yuan budget buy? — Good 10×50 binoculars, or a used starter refractor — but avoid brand-new sub-1,000 scopes.
- How heavy is an 8-inch Dob? — About 20 kg, splitting into tube and base — usually two trips.
- How many eyepieces do I need? — Two: a low-power finder (25 mm) and a high-power planetary (6–9 mm).
- Is a Barlow worth it? — Yes if it is a good one; a bad Barlow blurs rather than magnifies.
- Does the mount matter? — More than aperture — a shaky mount ruins any telescope.
- Can a phone photograph the Moon? — Yes — hold it to the eyepiece, manual exposure, low ISO, focus on the limb.
- What lens for nightscapes? — A fast wide lens (20 mm f/2.8), 15 s at ISO 3200 to start.
- Do I need a filter? — A UHC helps in a city, but it cannot undo light pollution.
- How many launch sites does China have? — Four national sites (Jiuquan, Xichang, Taiyuan, Wenchang) plus a commercial one on Hainan.
- How powerful is Long March 5? — About 25 t to LEO and 14 t to GTO.
- How many crew does Shenzhou carry? — Three, or two plus 300 kg of cargo.
- How many people are on Tiangong? — Three normally, six during handovers.
- How accurate is BeiDou? — About 2–3 m globally, centimetre-level with augmentation.
- How much did Chang'e 5 bring back? — 1,731 grams — China's first lunar sample return.
- What did Tianwen 1 achieve? — Orbit, landing and roving at Mars in a single 2021 mission.
- Which private Chinese launch companies exist? — LandSpace, i-Space, Galactic Energy, CAS Space and Orienspace among others.
- What is special about Zhuque-2? — It was the first methalox rocket to reach orbit, in 2023.
- Is Ceres-1 solid or liquid? — Solid — aimed at smallsat rideshares.
- Where does the data come from? — Launch Library 2 for launches, Celestrak for TLEs, astronomy-engine for the maths.
- How often is the site updated? — At build time; every page shows its snapshot age and flags stale past 12 hours.
- Why only three satellites? — ISS, Tiangong and Hubble are what people actually watch by eye, and their TLEs are stable.
- How accurate are pass predictions? — Good to the minute for fresh TLEs; a week-old set can be minutes off.
- Does the site use cookies or analytics? — No client-side analytics, no cookies, no third-party requests.
- Is my location uploaded? — No. It stays in your browser and is only used for local altitude maths.
- Does it work offline? — After the first load the Moon, planets and map still work — the maths is local.
- Can I see the source? — Yes — every number on a page traces back to a build script.
- Why is it Chinese by default? — Chinese readers are the audience; the English mirror lives under /en/.
- Will there be ads? — Later, once traffic is stable, and always labelled.
- What is LEO? — Low Earth orbit, roughly 160–2,000 km, where the stations live.
- What is GEO? — Geostationary orbit at 35,786 km, where a satellite hangs over one spot.
- What is GTO? — A transfer orbit to GEO; the satellite circularises itself afterwards.
- What is SSO? — Sun-synchronous orbit — same local time at each latitude, favoured by imaging satellites.
- What is a TLE? — Two 69-character lines encoding an orbit; the input to pass prediction.
- What is SGP4? — The model that propagates a TLE; error grows as the TLE ages.
- Why are orbits inclined? — The launch site's latitude sets the minimum; larger inclination reaches higher latitudes.
- Why does the ground track shift west? — Earth turns about 23° per 92-minute orbit, so each pass lands west of the last.
- Why do stars twinkle? — Atmospheric turbulence bends the light; planets are discs, so they twinkle less.
- Why is the sky blue? — Rayleigh scattering favours short blue wavelengths.
- Why are sunsets red? — Light crosses more air, blue is scattered out and red-orange remains.
- Why does the Moon look bigger near the horizon? — A perceptual illusion (Ponzo); the angular size is unchanged.
- Why isn't there an eclipse every month? — The Moon's orbit is tilted about 5°, so its shadow usually misses Earth.
- What is a solstice? — When the Sun reaches a tropic and day length peaks or bottoms out.
- What is an equinox? — Day and night are nearly equal as the Sun crosses the equator.
- Why is there polar night? — The 23.4° axial tilt keeps the Sun below the horizon all day at high latitude in winter.
- Why is the ISS brightest after dusk? — It is still sunlit while your sky is dark — maximum contrast.
- Why don't planets twinkle? — They are discs, not points, so turbulence averages out.
- Can you see the Moon in daylight? — Yes — a bright Moon is up in daytime, just low contrast.
- What do star colours mean? — Blue is hotter (10,000 K+), orange-red cooler (around 3,000 K).
- Why are there no stars in my Moon photo? — Exposure is set for the Moon; stars need a separate, much longer frame.
- Why does my telescope fog up? — The glass drops below the dew point; use a dew heater or wait for equilibrium.
- Why do stars trail in long exposures? — Earth's rotation; past 15 s you need a tracking mount.
- Why is a total lunar eclipse red? — Sunlight bent through Earth's atmosphere loses blue and reaches the Moon reddened.
- Why isn't the ISS visible every night? — The orbital plane is fixed while Earth turns, so windows drift in and out of your evening.
- Why is Hubble fainter than the ISS? — It is smaller and higher, with a fraction of the station's reflecting area.
- Why do some satellites flare? — Solar panels or flat surfaces catch the Sun at a specific angle.
- Why do rockets launch east? — To borrow Earth's eastward spin — about 465 m/s at the equator.
- Why don't polar launches go east? — A polar orbit needs a north-south track; eastward flight cannot reach that inclination.
- Why do rockets have stages? — Dropping empty structure lets the next stage reach orbit with a smaller engine.
- When is the fairing jettisoned? — Once above the dense atmosphere, usually past 100 km.
- Why does a booster relight for landing? — Reentry speed is far too high; it needs a burn to reach near-zero at touchdown.
- Why recover at sea? — Sea area is unrestricted and far from people; the ship can steam to the trajectory.
- Why are rockets mostly white? — White reflects sunlight, slows cryogen boil-off and shows ice.
- Why does the countdown hold? — A late check failed or range/weather is out of limits; it may resume or scrub.
- Why are some launches at night? — The window is set by orbital geometry, not daylight.
- Why does the ISS reboost? — Drag costs it a few km a year; cargo vehicles push it back up.
- Why do TLEs go stale? — Drag varies with solar activity, so errors accumulate until a new set is issued.
- Why is a TLE only two lines? — Legacy of punched cards: 69 fixed-width characters per line.
- Why does spaceflight use UTC? — It avoids time zones and daylight saving entirely.
- Why show a data timestamp? — Data ages; without a timestamp the reader cannot judge it.
- Why no ads yet? — Traffic is too small; content and trust come first.
- Can I see the ISS tonight? — Check the pass table here: a row labelled ISS means yes, in local time.
- How do I see a Starlink train? — One to three days after launch, look west 1–2 hours after sunset; they spread out after that.
- Can I see Andromeda by eye? — Yes, as a faint smudge near Cassiopeia under dark skies — not from a city.
- What does the ISS look like in a telescope? — Hard: it moves about 1° per second, so you chase it and see a bright cross.
- First time looking at Saturn? — Use 30–50×, wait until it is above 20°, avoid moonlight.
- How do I watch a shower alone? — Lie back and watch the whole sky; 20 quiet minutes is normal.
- How do I find a comet? — Check its magnitude and position, then sweep from a nearby bright star with binoculars.
- Can I see the ISS from a plane? — Sometimes, but the window is small and timing must be exact.
- How do I watch a solar eclipse safely? — Certified eclipse glasses or projection — ordinary sunglasses are not enough.
- How do I see sunspots safely? — Project through a certified solar filter — never look through the scope at the Sun.
- How does magnitude work? — A log scale: five magnitudes is a factor of 100, smaller is brighter.
- What are arcminutes and arcseconds? — 1° = 60′ = 3,600″; the Moon spans about 31′.
- How far is an astronomical unit? — About 149.6 million km — the mean Earth–Sun distance.
- How many km is a light-year? — About 9.46 trillion km.
- What is a parsec? — The distance at one arcsecond of parallax — about 3.26 light-years.
- What is the zenith? — Straight overhead, altitude 90°, the thinnest air.
- What is the meridian? — The north–zenith–south great circle; objects peak there.
- What is elongation? — Angular distance from the Sun for an inner planet.
- What is opposition? — A superior planet opposite the Sun, visible all night.
- What is a conjunction? — Two bodies appearing close on the sky.
- How do I photograph an ISS pass? — Wide lens at f/2.8, 15–20 s frames, then stack the streak.
- What settings for the Milky Way? — 20 mm f/2.8, 15 s, ISO 3200, manual focus on a bright star.
- What settings for the Moon? — ISO 100, f/8, 1/125 s — expose for the Moon, not the sky.
- How do I shoot star trails? — Sixty 30-second frames stacked, or a 10-minute exposure.
- How do I stop lens dew? — A dew heater or a longer hood; let gear acclimatise outside first.
- How far from a city is dark enough? — Usually 50 km out, facing away — worth 2–3 Bortle classes.
- What should I pack for a dark-site trip? — Red torch, charts, warm layers, hot drink, spare batteries and a folding chair.
- Why observe at altitude? — Thinner, drier air means better transparency — but mind altitude sickness.
- How do I clean a lens? — Blower first, then lens tissue in one direction — never dry-scrub.
- How should I store a telescope? — Dry, dark, tube horizontal, batteries out for long storage.
- How do I collimate a reflector? — Cheshire or laser: secondary first, then primary, then check a star.
- Can I wash a mirror? — Avoid it; if you must, distilled water and lint-free cotton, at your own risk.
- Why do batteries die in the cold? — Cold slows lithium chemistry; keep spares in a pocket.
- How do I align a GoTo mount? — Two- or three-star alignment with widely separated bright stars.
- How do I find Polaris? — Extend the line through the Big Dipper's pointer stars about five times.
- How do I find south? — Extend the Southern Cross's long axis about 4.5 times.
- What is the best season for stargazing? — Autumn and winter give long, transparent nights; summer has the Milky Way but short nights.
- Can I observe in the rainy season? — Watch the cloud forecast for gaps, and aim at bright targets like the Moon.
- How do I plan around the Moon? — Check the phase, avoid ±5 days of full moon for deep sky, use full moon for the Moon itself.
- When are planets best? — Near opposition or greatest elongation: Mars every 26 months, Jupiter and Saturn yearly.
- Why do two apps disagree on a pass? — Different TLE ages and models — tens of seconds is normal.
- Why are some passes so low? — The ground track misses you, so peak altitude stays under 20°.
- Why two passes in one night? — A 92-minute orbit can put two consecutive passes in darkness.
- Can the ISS suddenly brighten? — Yes — panel glint can add one or two magnitudes.
- Why is Hubble hard to see? — Smaller and fainter (magnitude 1–2), needing precise predictions and dark sky.
- Why did Moon landings stop? — Budget and priorities shifted; Artemis is restarting crewed landings.
- Will the space station fall? — It needs periodic reboosts; at end of life it is deorbited deliberately.
- How many satellites are in orbit? — Around 11,000 active, more than half of them Starlink.
- How bad is space debris? — About 36,000 objects over 10 cm, and perhaps a million over 1 cm.
- What is the Kessler syndrome? — A cascade where collisions breed debris until an orbit becomes unusable.
- Why is there a fireball on reentry? — Compressed air becomes plasma above 2,000 K.
- Why is there a sonic boom? — Shock waves form when a vehicle passes the speed of sound.
- What is the white cloud at launch? — Mostly steam from the water deluge meeting the exhaust.
- Why are night launches so bright? — The plume lights the cloud deck and smoke column for tens of kilometres.
- Why the dense counting at T-10? — The last ten seconds are the critical sequence; aborts must complete before T-0.
- Why hold at T-9? — The automatic sequence takes over there and aborts on any out-of-limit parameter.
- Why does the ISS sometimes hold six? — Handover overlap between an arriving and a departing crew.
- Do astronauts get taller? — Yes — discs expand, adding 3–5 cm, and it reverses on return.
- Why do astronauts exercise daily? — Bone density drops about 1% a month without loading exercise.
- Why are CubeSats popular? — Standard sizes and rideshare slots bring costs into the hundreds of thousands of yuan.
- Why are launch prices falling? — Reuse and volume: Falcon 9 boosters have flown over 20 times each.