Yeah, temperature is a big one. As is voltage supply stability.
If you want really good short & medium term stability, it's hard (expensive) to do better than an Oven-Controlled Crystal Oscillator (OCXO). An OCXO has a crystal in an insulated chamber with a heater and a thermocouple. A control circuit uses the heater to keep the chamber at a consistent temperature. Over long periods these still drift.
A cheaper alternative is a Temperature Compensated Crystal Oscillator (TCXO), that combines a control circuit with a crystal, a temperature sensor, and a ROM. The ROM contains a table of frequency errors that crystal had across temperature. The control circuit senses the temperature, reads the error value from the ROM, and tries to correct the crystal's oscillation frequency to compensate for that amount of error. Less accurate and less stable than an OCXO, but much smaller, much lower power, and much cheaper.
For longer-term stability you want an atomic clock, probably a Cesium clock or Hydrogen Maser. GNSS (GPS, Galileo, GLONASS, Baidu, etc) satellites have atomic clocks on board, and broadcast that time. GNSS is based around very precisely measuring the differences in received times from various clocks to calculate location, so it can also be used as a way to get a local time reference with good long-term stability. Unfortunately GNSS does have rather poor short-term stability compared to an OCXO, so GNSS alone isn't perfect. But it's common and reasonably inexpensive to create a "GPS-Disciplined OCXO" where a GPS unit corrects for the long-term drift of an OCXO, but the OCXO provides the actual output signal (thus gaining the good short & medium-term stability).
The NIST SP 1065 Handbook of Frequency Stability Analysis[1] is a go-to text on measuring clock sources.
If you want really good short & medium term stability, it's hard (expensive) to do better than an Oven-Controlled Crystal Oscillator (OCXO). An OCXO has a crystal in an insulated chamber with a heater and a thermocouple. A control circuit uses the heater to keep the chamber at a consistent temperature. Over long periods these still drift.
A cheaper alternative is a Temperature Compensated Crystal Oscillator (TCXO), that combines a control circuit with a crystal, a temperature sensor, and a ROM. The ROM contains a table of frequency errors that crystal had across temperature. The control circuit senses the temperature, reads the error value from the ROM, and tries to correct the crystal's oscillation frequency to compensate for that amount of error. Less accurate and less stable than an OCXO, but much smaller, much lower power, and much cheaper.
For longer-term stability you want an atomic clock, probably a Cesium clock or Hydrogen Maser. GNSS (GPS, Galileo, GLONASS, Baidu, etc) satellites have atomic clocks on board, and broadcast that time. GNSS is based around very precisely measuring the differences in received times from various clocks to calculate location, so it can also be used as a way to get a local time reference with good long-term stability. Unfortunately GNSS does have rather poor short-term stability compared to an OCXO, so GNSS alone isn't perfect. But it's common and reasonably inexpensive to create a "GPS-Disciplined OCXO" where a GPS unit corrects for the long-term drift of an OCXO, but the OCXO provides the actual output signal (thus gaining the good short & medium-term stability).
The NIST SP 1065 Handbook of Frequency Stability Analysis[1] is a go-to text on measuring clock sources.
[1] https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpubli...