```ts
/** Private helper function to validate temperature inputs */
const validateCelsius = (celsius: number): boolean => {
  if (typeof celsius !== "number") {
    throw new Error("celsius must be a finite number");
  }
  return Number.isFinite(celsius);
};

/** The temperature in degrees Fahrenheit. */
export function celsiusToFahrenheit(celsius: number): number {
  if (!validateCelsius(celsius)) {
    throw new Error("celsius must be a finite number");
  }
  if (celsius < -273.15) {
    throw new Error("celsius must be at or above absolute zero");
  }
  return celsius * 1.8 + 32;
}

/** The temperature in kelvin. */
export function celsiusToKelvin(celsius: number): number {
  if (!validateCelsius(celsius)) {
    throw new Error("celsius must be a finite number");
  }
  if (celsius < -273.15) {
    throw new Error("celsius must be at or above absolute zero");
  }
  return celsius + 273.15;
}
```