```ts
/* eslint-disable @typescript-eslint/no-shadow */
function validateCelsiusInput(celsius: number): void {
  if (typeof celsius !== "number" || !Number.isFinite(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");
  }
}

declare module solution {
  export function celsiusToFahrenheit(celsius: number): number;
  export function celsiusToKelvin(celsius: number): number;
}

/** The temperature in degrees Fahrenheit. */
function celsiusToFahrenheit(celsius: number): number {
  validateCelsiusInput(celsius);
  return celsius * 1.8 + 32;
}

/** The temperature in kelvin. */
export function celsiusToKelvin(celsius: number): number {
  validateCelsiusInput(celsius);
  return celsius + 273.15;
}
```